Integrated capture and conversion of CO2 to methane, methanol, or methanol and glycol

ABSTRACT

A process for producing methane or methanol includes combining a hydrogenation catalyst, hydrogen, and CO 2  with a condensed phase solution comprising an amine under conditions effective to form methane or methanol, and water. A process for coproduction of methanol and a glycol includes combining an epoxide, a hydrogenation catalyst, hydrogen, and CO 2  with a condensed phase solution comprising an amine under conditions effective to form methanol and a glycol.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No. 16/928,649, filed Jul. 14, 2020, which claims the benefit of the earlier filing dates of U.S. Provisional Application No. 63/038,464, filed Jun. 12, 2020, and U.S. Provisional Application No. 62/874,383, filed Jul. 15, 2019, each of which is incorporated by reference herein in its entirety.

ACKNOWLEDGMENT OF GOVERNMENT SUPPORT

This invention was made with Government support under Contract DE-AC05-76RL01830 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.

FIELD

This invention is directed to an integrated process for capture and conversion of CO₂ to methane, methanol or a combination of methanol and glycol.

SUMMARY

This disclosure concerns embodiments of an integrated process for capture and conversion of CO₂ to methane or methanol, or concurrently to methanol and a glycol. In some embodiments, the process is a one-step process. In other embodiments, the process is a two-step process. The process may be a one-pot process. In some embodiments, the process is continuous.

In some embodiments, an integrated process includes producing methanol by combining a hydrogenation catalyst, hydrogen, and CO₂ with a condensed phase comprising an amine under conditions effective to provide a reaction between the hydrogen and CO₂ to form methanol and water. The methanol and water are separated. In certain embodiments, the conditions effective to form methanol and water include (i) a temperature T_(M) within a range of from 50-180° C., or (ii) an initial pressure P_(M) within a range of from 1 MPa to 10 MPa, or (iii) a time t_(M) within a range of from 3 seconds to 36 hours, or (iv) any combination of (i), (ii), and (iii). In certain implementations, the hydrogenation catalyst comprises Cu/ZnO/Al₂O₃.

In some embodiments, an integrated process includes producing methane by combining a hydrogenation catalyst, hydrogen, and CO₂ with a condensed phase comprising an amine under conditions effective to provide a reaction between the hydrogen and CO₂ to form methane and water. The methane is separated from the water and the amine. In certain embodiments, the conditions effective to form methanol and water include (i) a temperature T_(M) of 100-200° C., or (ii) an initial pressure P_(M) of 1 MPa to 10 MPa, or (iii) a time t_(M) of 3 seconds to 36 hours, or (iv) any combination of (i), (ii), and (iii). In certain implementations, the hydrogenation catalyst comprises a metal, wherein the metal is Ru, Ni, Fe, Co, Rh, or a combination thereof; and a support, wherein the support comprises an oxide or carbon.

In some embodiments, an integrated process includes producing methanol and a glycol by combining an epoxide, a hydrogenation catalyst, hydrogen, and CO₂ with a condensed phase comprising an amine under conditions effective to provide a reaction between the epoxide, hydrogen, and CO₂ to form methanol and the glycol. In some embodiments, the conditions effective to form methanol and the glycol include (i) a temperature T_(MG) within a range of from 50° C. to 170° C., or (ii) an initial pressure P_(MG) within a range of from 3 MPa to 10 MPa, or (iii) a time t_(MG) within a range of from 3 seconds to 36 hours, or (iv) any combination of (i), (ii), and (iii).

The epoxide comprises

and the glycol comprises R—CH(OH)—CH(R′)(OH), where R and R′ independently are H or aliphatic, or R and R′ together with the carbon atoms to which they are attached form a cycloaliphatic ring. In some embodiments, R is methyl and R′ is H.

In any of the foregoing or following embodiments, the process may further include combining the epoxide, the CO₂, and the condensed phase under cycloaddition conditions effective to form a cyclic carbonate, thereby producing a cyclic carbonate-containing mixture, and subsequently combining the cyclic carbonate-containing mixture with the hydrogenation catalyst and hydrogen under conditions effective to concurrently form methanol and the glycol. Alternatively, in any of the foregoing or following embodiments, the process may further include forming a mixture by combining the epoxide, the hydrogen catalyst, and the CO₂ with the condensed phase, exposing the mixture to cycloaddition conditions effective to form a cyclic carbonate, thereby producing a cyclic carbonate-containing mixture, and combining the cyclic carbonate-containing mixture with hydrogen under conditions effective to form methanol and the glycol. In some embodiments, the cycloaddition conditions effective to form a cyclic carbonate comprise (i) a temperature T_(C) within a range of from 25° C. to 180° C., or (ii) an initial pressure P_(C) within a range of from 0.1 MPa to 2 MPa, or (iii) a time t_(C) within a range of from 3 seconds to hours, or (iv) any combination of (i), (ii), and (iii).

In any of the foregoing or following embodiments, the amine may comprise a 1° amine group, a 2° amine group, a 3° amine group, or any combination thereof. In any of the foregoing or following embodiments, the CO₂ may be adsorbed, absorbed, covalently, or ionically bound to the amine. In some embodiments, the amine comprises a polyamine, a tertiary amine, a compound according to Formula I, or any combination thereof, the compound according to Formula I having a structure R¹(R²)N-L¹-NH—R³ where each of R¹ and R² independently is aliphatic or cycloaliphatic or R¹ and R² together with the nitrogen to which they are attached form a heterocyclyl ring, L¹ is aliphatic or cycloaliphatic or L¹ and R¹ together with the nitrogen to which they are attached form a heterocyclyl ring, and R³ is aliphatic, cycloaliphatic, cycloalkylalkyl or alkoxyalkyl.

In any of the foregoing embodiments, the condensed phase may be a condensed phase comprising the CO₂ prior to combination with the other components. In such embodiments, the condensed phase may be prepared by contacting a gas stream comprising CO₂ with an amine-containing solvent or solution prior to combing the condensed phase solution with the remaining components. In some embodiments, contacting the gas stream with the amine-containing solvent or solution is performed (i) at a temperature T′ within a range of from 20° C. to 60° C., or (ii) at an initial pressure P′ within a range of from 0.1 MPa to 5 MPa, or (iii) both (i) and (ii).

In any of the foregoing or following embodiments, the hydrogenation catalyst may comprise known materials to catalyze hydrogenation such as (i) precious metals Pt, Pd, Rh, Ir, and Ru, base metals Ni, Co, Fe, Cu, and Zn, and catalysts supported on materials such as carbon, Al₂O₃, In₂O₃, CeO₂, TiO₂, SiO₂, ZrO₂, magnesium aluminum spinels (e.g., MgO—Al₂O₃), chromite (e.g., FeCr₂O₄), or any combination thereof; or (ii) a pincer ligand; or (iii) both (i) and (ii).

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a flow diagram of one exemplary embodiment of a process for converting CO₂ to methanol.

FIG. 2 is a flow diagram of another exemplary embodiment of a process for converting CO₂ to methanol.

FIGS. 3A and 3B are simplified schematic diagrams of an exemplary apparatus for performing the process of FIG. 2.

FIG. 4 is a flow diagram of one exemplary embodiment of a process for converting CO₂ to methanol and a glycol.

FIG. 5 is a flow diagram of another exemplary embodiment of a process for converting CO₂ to methanol and a glycol.

FIG. 6 is a flow diagram of another exemplary embodiment of a process for converting CO₂ to methanol and a glycol.

FIG. 7 is a flow diagram of another exemplary embodiment of a process for converting CO₂ to methanol and a glycol.

FIGS. 8A and 8B are simplified schematic diagrams of an exemplary apparatus for performing the process of FIG. 5.

FIGS. 9A and 9B are simplified schematic diagrams of an exemplary apparatus for performing the process of FIG. 7.

FIG. 10 is an exemplary reaction scheme showing conversion of CO₂ to N-methylated products, methane, or methanol under different conditions.

FIG. 11 is a flow diagram of one exemplary embodiment of a process for converting CO₂ to methane.

FIG. 12 is a flow diagram of another exemplary embodiment of a process for converting CO₂ to methane.

FIGS. 13A and 13B are simplified schematic diagrams of an exemplary apparatus for performing the process of FIG. 12.

FIG. 14 shows reaction schemes for CO₂ capture using different amines.

FIG. 15 is a bar graph showing methanol yields from CO₂ using various alcohols and amines.

FIG. 16 illustrates a potential reaction mechanism for conversion of CO₂ to methanol in the presence of triethylamine and ethanol.

FIG. 17 is a bar graph showing catalytic activity of a recycled Cu/ZnO/Al₂O₃ catalyst over three cycles.

FIG. 18 is a graph percentage of ethyl carbonate formed with respect to total amine content vs. temperature.

FIG. 19 shows in situ ¹³C MAS NMR of triethylamine:ethanol 1:10 reacted with ¹³CO₂ and H₂ in the presence of a Cu/ZnO/Al₂O₃ catalyst at 170° C. over one hour.

FIG. 20 shows in situ ¹³C MAS NMR of triethylamine:ethanol 1:10 reacted with ¹³CO₂ and H₂ in the presence of a Pd/ZnO catalyst at 170° C. over 15 h.

FIG. 21 is a graph showing the rate of hydrogenation of in situ-formed propylene carbonate in the presence of three ruthenium-based catalysts.

FIG. 22 is a graph showing the rate of cyclic carbonate formation from reaction of CO₂ and propylene oxide in the presence of two Ru-based catalysts and in the absence of a hydrogenation catalyst.

FIG. 23 is a ¹H NMR spectrum of a reaction mixture after sequential addition of CO₂ and H₂ to a propylene oxide-PEI₆₀₀-tetrahydrofuran mixture in the presence of a Ru-based catalyst at 140° C.

FIG. 24 shows one potential mechanism for formation of methanol and 1,2-propylene glycol from a reaction of CO₂, propylene oxide, and H₂ in the presence of a Ru-based catalyst.

FIG. 25 shows in situ ¹³C MAS NMR of a one-step, one-pot reaction of propylene oxide with CO₂ and H₂ in the presence of a Ru-based catalyst.

FIG. 26 is a diagram showing the energetics for coproduction of methanol and propylene glycol. Relative energies given are enthalpy of reactions. Routes (a) and (b) are conventional and the disclosed coproduction routes, respectively.

FIG. 27 is a schematic diagram illustrating the capture of CO₂ by N-(2-ethoxyethyl)-3-morpholinopropan-1-amine (EEMPA) and hydrogenation to form various products via a formamide intermediate.

FIG. 28 is an NMR spectrum showing methanation of EEMPA-captured CO₂ in the presence of a Ru/Al₂O₃ catalyst.

FIG. 29 is a graph showing selectivity and CO₂ conversion vs. WHSV in fixed-bed experiments in EEMPA solvent with 10 wt % CO₂ load over 5 wt % Ru/Al₂O₃ catalyst at 170° C.

DETAILED DESCRIPTION

Embodiments of an integrated process for capture and conversion of CO₂ to methanol or to methanol and a glycol are disclosed. Embodiments of an integrated process for capture and conversion of CO₂ to methane also are disclosed.

Methanol is a commodity chemical that can be used as a feedstock to produce olefins, ethers, fuel blends, acetic acid, and other products. Glycols, such as ethylene glycol and propylene glycol, are used as automotive antifreeze, chemical feedstocks for polyester production, and for other miscellaneous applications.

Industrially, methanol typically is produced from a syngas mixture in the presence of a Cu-based catalyst at high temperature and high pressure as shown below. The reaction byproduct, water, is separated from methanol by distillation and is not utilized in the process, which lowers the net theoretical atom efficiency to ˜73%.

Glycols are typically produced by hydrolysis of epoxide in excess water under acidic conditions at high temperature (>150° C.). In addition, the hydrolysis product stream is often contaminated with oligomers of glycols because the product glycol reacts faster with epoxide than water. The use of excess water (˜20-fold molar excess) is necessary to reduce the formation of higher homologues, and there is an energy penalty associated with separation of glycol(s) from excess water and oligomers.

In the case of ethylene oxide-to-ethylene glycol conversion, a two-step Shell OMEGA process (shown below) involving the formation of a cyclic carbonate and subsequent hydrolysis of the cyclic carbonate to ethylene glycol is practiced industrially to improve the selectivity for the glycol. However, the process still involves an energy-intensive separation process to remove excess water from the product.

Another known two-step process reacts CO₂ with an epoxide (R is H or alkyl) to form a cyclic carbonate, which is reduced with a hydrosilane and N-tetrabutylammonium fluoride (TBAF) to form a glycol and methanol (Bobbink et al., ACS Sustainable Chem. Eng. 2018, 6:12119-12123). A major disadvantage is the silane waste produced.

Embodiments of the foregoing processes suffer from additional disadvantages including a requirement for added water and, in some cases, an added base. The foregoing processes also may require base-assisted aqueous work-up, thereby complicating the separation process.

In a conventional process for methane production, flue gas is first sent to an aqueous amine-based carbon capture system, where CO₂ in the flue gas is captured in the absorber and released in the stripper at about 0.2 MPa (2 bar) with thermal regeneration. The pure CO₂ from the stripper is compressed and mixed with hydrogen before sent to the methanation reactor, where the Sabatier reaction (below), takes place at 350° C. and 3 MPa (30 bar) with the Ru/Al₂O₃ catalyst. The Sabatier reaction is exothermic, of which most of the reaction heat is used to supply the energy required in the CO₂ stripper, while the remaining is used for power generation. The hot product from the methanation reactor is used to preheat reactor feed. By-product water is removed by condensation. The unconverted hydrogen is recovered in a membrane separator and recycled back to the reactor. CO₂₊₄H₂↔CH₄+2H₂O ΔH=−165 kJ/mol

Embodiments of the disclosed integrated processes provide advantages over the foregoing processes. In some embodiments, the integrated process reduces energy requirements (e.g., by operating at a lower temperature, requiring less compression of CO₂), simplifies equipment, eliminates one or more separations of excess reagents and/or byproducts, reduces waste, and/or improves atom efficiency. Embodiments of the discloses integrated process do not require added water and/or form products that can be simply separated, e.g., by distillation. Instead, in some embodiments, reactant water is formed and consumed in situ.

In some embodiments, an integrated process for producing methanol or methane includes combining a hydrogenation catalyst, hydrogen, and CO₂ with a condensed phase comprising an amine under conditions effective to form methanol or methane and water. In some embodiments, an integrated process for coproducing methanol and a glycol includes combining an epoxide, a hydrogenation catalyst, hydrogen, and CO₂ with a condensed phase comprising an amine under conditions effective to form methanol and a glycol.

In some embodiments, the integrated process is a one-step process. In other embodiments, the process is a two-step process. The process may be continuous. The CO₂ and H₂ utilized in embodiments of the disclosed processes may be obtained from any suitable source. In some embodiments, the CO₂ is obtained from landfill gases, wastewater treatment gases, manure gas sources, or other industrial processes. CO₂ is captured using a condensed phase amine. In contrast to existing technologies, subsequent conversion to methane, methanol, or methanol and glycol is performed in the condensed phase amine. Advantageously, embodiments of the disclosed processes eliminate the need to separate and compress CO₂ for gas-phase hydrogenation and the accompanying energy requirement for the separation and compression. In some embodiments, hydrogenation is performed in the condensed phase at a lower temperature than those required for gas phase hydrogenation. These advantages render embodiments of the disclosed integrated capture and conversion process more economically viable than existing technologies.

I. Definitions

The following explanations of terms and abbreviations are provided to better describe the present disclosure and to guide those of ordinary skill in the art in the practice of the present disclosure. As used herein, “comprising” means “including” and the singular forms “a” or “an” or “the” include plural references unless the context clearly dictates otherwise. The term “or” refers to a single element of stated alternative elements or a combination of two or more elements, unless the context clearly indicates otherwise.

Unless explained otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure, suitable methods and materials are described below. The materials, methods, and examples are illustrative only and not intended to be limiting. Other features of the disclosure are apparent from the following detailed description and the claims.

The disclosure of numerical ranges should be understood as referring to each discrete point within the range, inclusive of endpoints, unless otherwise noted. Unless otherwise indicated, all numbers expressing quantities of components, molecular weights, percentages, temperatures, times, and so forth, as used in the specification or claims are to be understood as being modified by the term “about.” Accordingly, unless otherwise implicitly or explicitly indicated, or unless the context is properly understood by a person of ordinary skill in the art to have a more definitive construction, the numerical parameters set forth are approximations that may depend on the desired properties sought and/or limits of detection under standard test conditions/methods as known to those of ordinary skill in the art. When directly and explicitly distinguishing embodiments from discussed prior art, the embodiment numbers are not approximates unless the word “about” is recited.

Although there are alternatives for various components, parameters, operating conditions, etc. set forth herein, that does not mean that those alternatives are necessarily equivalent and/or perform equally well. Nor does it mean that the alternatives are listed in a preferred order unless stated otherwise.

Definitions of common terms in chemistry may be found in Richard J. Lewis, Sr. (ed.), Hawley's Condensed Chemical Dictionary, published by John Wiley & Sons, Inc., 2016 (ISBN 978-1-118-13515-0).

Aliphatic: A substantially hydrocarbon-based compound, or a radical thereof (e.g., C₆H₁₃, for a hexane radical), including alkanes, alkenes, alkynes, including cyclic versions thereof (cycloaliphatic), and further including straight- and branched-chain arrangements, and all stereo and position isomers as well. Unless expressly stated otherwise, an aliphatic group contains from one to twenty-five carbon atoms; for example, from one to fifteen, from one to ten, from one to six, or from one to four carbon atoms. The term “lower aliphatic” refers to an aliphatic group containing from one to ten carbon atoms. An aliphatic chain may be substituted or unsubstituted. Unless expressly referred to as an “unsubstituted aliphatic,” an aliphatic group can either be unsubstituted or substituted.

Alkoxyalkyl: A radical having the general formula —R—O—R′ where R and R′ independently are alkyl.

Amine: A compound including an amino group —N(R)R′ where R and R′ are independently hydrogen, alkyl, heteroalkyl, haloalkyl, aliphatic, heteroaliphatic, aryl (such as optionally substituted phenyl or benzyl), heteroaryl, alkylsulfano, or other functionality. A 1°, or primary, amine includes an —NH₂ group. A 2°, or secondary, amine includes an —N(H)R group. A 3°, or tertiary, amine includes an —N(R)R′ group. A polyamine is an organic compound having more than two amino groups.

Aromatic or aryl: A monovalent aromatic carbocyclic group of, unless specified otherwise, from 6 to 15 carbon atoms having a single ring (e.g., phenyl) or multiple condensed rings in which at least one ring is aromatic (e.g., quinoline, indole, benzodioxole, and the like), provided that the point of attachment is through an atom of an aromatic portion of the aryl group and the aromatic portion at the point of attachment contains only carbons in the aromatic ring. If any aromatic ring portion contains a heteroatom, the group is a heteroaryl and not an aryl. Aryl groups are monocyclic, bicyclic, tricyclic or tetracyclic.

Catalyst: A substance, usually present in small amounts relative to reactants, which increases the rate of a chemical reaction without itself being consumed or undergoing a chemical change. A catalyst also may enable a reaction to proceed under different conditions (e.g., at a lower temperature) than otherwise possible.

Condensed phase: The term “condensed phase” refers to a liquid or solid phase, including liquid/liquid, liquid/solid, and solid/solid solutions and mixtures.

Cyclic carbonate: A compound having a general formula

where R¹-R⁴ independently are H or aliphatic. In some disclosed embodiments, R¹ is aliphatic, such as C₁-C₅ alkyl, and R²-R⁴ are H.

Cycloaddition: As used herein, cycloaddition refers to addition of CO₂ to an epoxide, resulting in a cyclic carbonate.

Cycloalkylalkyl: A radical having a general formula —R—R′ where R is alkyl and R′ is cycloalkyl, e.g., —CH₂-cycloalkyl.

Epoxide: A cyclic ether with a 3-membered ring having a general formula

where R¹-R⁴ independently are H or aliphatic, e.g., C₁-C₅ alkyl. In some cases, R² and R⁴ together with the carbon atoms to which they are attached form a cycloaliphatic ring.

Glycol: A dihydric alcohol (a diol) in which the two hydroxyl groups are on different carbon atoms, typically two adjacent carbon atoms.

Heteroaliphatic: An aliphatic compound or group having at least one carbon atom in the chain and at least one heteroatom, i.e., one or more carbon atoms has been replaced with an atom having at least one lone pair of electrons, typically nitrogen, oxygen, phosphorus, silicon, or sulfur. Heteroaliphatic compounds or groups may be substituted or unsubstituted, branched or unbranched, cyclic or acyclic, and include “heterocycle”, “heterocyclyl”, “heterocycloaliphatic”, or “heterocyclic” groups.

Heteroaryl: An aromatic compound or group having at least one heteroatom, i.e., one or more carbon atoms in the ring has been replaced with an atom having at least one lone pair of electrons, typically nitrogen, oxygen, phosphorus, silicon, or sulfur (e.g., pyridine).

Heteroaryl compounds may be substituted (e.g., aminopyridine) or unsubstituted.

Heterocyclyl ring: Refers to a closed-ring compound, or radical thereof as a substituent bonded to another group, particularly other organic groups, where at least one atom in the ring structure is other than carbon, and typically is oxygen, sulfur and/or nitrogen.

Pincer ligand: A tridentate chelating ligand that forms three bonds to a metal cation to form a complex. Typically, a pincer ligand binds to three adjacent coplanar sites on the metal cation in a meridional configuration.

WHSV: Weight hourly space velocity. As used herein, WHSV is defined as the weight of CO₂ flowing per weight of catalyst per hour.

II. Condensed Phase Comprising an Amine

Embodiments of the disclosed integrated process include a condensed phase comprising an amine. In any of the following embodiments, the condensed phase may be an amine in a liquid state, a solution comprising an amine and a solvent in a liquid state, a solid state (e.g., an amine supported on SiO₂), or a slurry comprising a solid amine and a solvent in a liquid state (e.g., chitosan/polyethylene glycol). In some embodiments, the solvent is nonaqueous. The amine comprises a 1° amine group, a 2° amine group, a 3° amine group, or any combination thereof.

In any of the foregoing or following embodiments, the amine may comprise a polyamine, a tertiary amine, an alkanolamine, an aminopyridine, a diamine compound according to Formula I, or any combination thereof, the compound according to Formula I having a structure R¹(R²)N-L¹-NH—R³ where each of R¹ and R² independently is aliphatic or cycloaliphatic or R¹ and R² together with the nitrogen to which they are attached form a heterocyclyl ring, L¹ is aliphatic or cycloaliphatic or L¹ and R¹ together with the nitrogen to which they are attached form a heterocyclyl ring, and R³ is aliphatic, cycloaliphatic, cycloalkylalkyl or alkoxyalkyl. In certain embodiments, the amine is not an amidine, since amidines may decompose under the reaction conditions.

In some embodiments, the condensed phase consists of the amine or consists essentially of the amine. By “consists essentially of” or “consists of” is meant that no additional solvent is affirmatively added to the amine. By “consists essentially of” is further meant that, prior to utilization in the integrated process, the condensed phase comprises at least 95 wt % of the amine. In other embodiments, the condensed phase is a solution comprising the amine and a solvent. In one embodiment, the solvent is a nonaqueous solvent. Exemplary nonaqueous solvents include, but are not limited to, alcohols (e.g., alkanols) and cyclic ethers (e.g., tetrahydrofuran). In some embodiments, the condensed phase is water-lean, i.e., comprising 0 wt % to 10 wt % water, such as 0 wt % to 5 wt % water, or 0 wt % to 2 wt % water. In an independent embodiment, the condensed phase is a solid state comprising the amine on a solid support, such as an amine-functionalized solid support.

In some embodiments, the condensed phase is a solution or a slurry comprising a solvent and from 1 mol % to 95 mol % of a tertiary amine or aminopyridine (with the remainder being the solvent), such as from 1 mol % to 75 mol %, 1 mol % to 50 mol %, 1 mol % to 20 mol %, 5 mol % to 20 mol %, or 5 mol % to 10 mol % of the tertiary amine or aminopyridine, or from 0.1 mol % to 2 mol % of a polyamine, such as from 0.2-1 mol % of the polyamine, or any combination thereof. In an independent embodiment, the condensed phase consists of or consists essentially of a polyamine or a compound according to Formula I.

In some embodiments, the amine is a polyamine. The polyamine may be, for example, a polyimine. In certain embodiments, the polyamine is polyethyleneimine (PEI) with a repeating unit of —[CH₂CH₂N(H)]_(n)— where n is an integer. The PEI may be linear or branched. Some branched PEIs are liquids including primary, secondary, and tertiary amino groups. One exemplary structure of a branched PEI having 1°, 2°, and 3° amine groups in an approximate ratio of 25:50:25 is shown below:

Liquid branched PEIs are soluble in water, lower alcohols (e.g., C₁-C₅ alcohols), glycols, and tetrahydrofuran (THF). In some embodiments, the PEI is a branched PEI. In certain embodiments, the branched PEI has a number average molecular weight M_(n) within a range of from 500-250000 Da, such as from 500-10000 Da, 500-2000 Da, or 500-1000 Da. In any of the foregoing embodiments, the condensed phase may be a solution comprising the polyamine and a solvent. In some embodiments, the condensed phase comprises PEI and a nonaqueous solvent. In some examples, the condensed phase comprised branched PEI (M_(n)˜600) and tetrahydrofuran.

In some embodiments, the amine is a tertiary amine. The tertiary amine may be, for example, a trialkylamine, a tertiary alkyl-substituted diamine, a tertiary alkanolamine, such as a dialkylalkanolamine, a tertiary cycloaliphatic amine, or a tertiary aromatic or heteroaromatic amine, such as a tertiary alkyl-substituted aminopyridine. In certain embodiments, the alkyl groups are C₁-C₃ alkyl groups. Exemplary tertiary amines include, but are not limited to, triethylamine, tetramethylethylenediamine, diethylethanolamine, N,N-diisopropylethylamine, 4-dimethylaminopyridine, and combinations thereof. In certain embodiments, the condensed phase is a solution comprising a trialkylamine and an alcohol. For example, the condensed phase may comprise triethylamine and ethanol. In some embodiments, the condensed phase comprises a trialkylamine and a C₁-C₃ alkanol in an amine:alkanol molar ratio of from 1:5 to 1:15, such as a molar ratio of from 1:8 to 1:12. In certain examples, the condensed phase comprised triethylamine and ethanol in a molar ratio of 1:10.

In some embodiments, the amine is a substituted pyridine, wherein the substituent comprises at least one amine group. The amine group may be directly attached to the pyridine ring, such as 4-dimethylaminopyridine. In some examples, the nitrogen of the amine is not directly attached to the pyridine ring. Suitable pyridines include, but are not limited to:

In some embodiments, the amine is a compound according to Formula I: R¹(R²)N-L¹-NH—R³   Formula I.

Compounds according to Formula I also may referred to as carbon dioxide binding organic liquids (CO₂BOLs). With respect to Formula I, each of R¹ and R² independently is aliphatic, preferably alkyl, such as C₁₋₆alkyl, C₁₋₄alkyl, C₁₋₃alkyl, or C₁₋₂alkyl; cycloaliphatic, preferably cycloalkyl, such as C₃₋₇cycloalkyl or C₃₋₄cycloalkyl, and may be cyclopropyl; or R¹ and R² together with the nitrogen to which they are attached, form a heterocyclyl ring, such as an non-aromatic heterocyclyl ring, preferably a 5- or 6-membered heterocyclyl ring and optionally comprising one or more additional heteroatoms, such as 1 or 2 heteroatoms selected from oxygen, nitrogen or sulfur, and/or optionally substituted with alkyl, such as C₁₋₄alkyl. Alternatively, R¹ may form a heterocyclyl moiety, with L¹, such as a non-aromatic heterocyclyl moiety, preferably a 5- or 6-membered heterocyclyl moiety. In such embodiments, R² is aliphatic, preferably alkyl, such as C₁-C₆alkyl, C₁₋₄alkyl, C₁₋₃alkyl, or C₁₋₂alkyl, or cycloalkyl, such as C₃₋₇cycloalkyl.

In some embodiments, each of R¹ and R² independently is linear alkyl or branched alkyl, such as C₁₋₆linear alkyl, or C₃₋₆branched alkyl. Exemplary linear alkyl moieties include, but are not limited to methyl, ethyl, n-propyl or n-butyl, and exemplary branched alkyl moieties include, but are not limited to, isopropyl, tert-butyl, iso-butyl, or sec-butyl. And R¹ and R² may be the same or different. In other embodiments, R¹ and R² together with the nitrogen to which they are attached form a non-aromatic heterocyclyl moiety, such as morpholine, thiomorpholine, piperidine, pyrrolidine, or piperazine, optionally substituted with C₁₋₄alkyl, typically, methyl, ethyl, isopropyl, or tert-butyl.

L¹ is aliphatic, preferably alkyl, such as C₂₋₄alkyl or C₂₋₃alkyl; cycloaliphatic, preferably cycloalkyl, such as C₅₋₇cycloalkyl; or L¹ and R¹ together with the nitrogen to which they are attached form a non-aromatic heterocyclyl ring, such as a 5-, 6-, or 7-membered heterocyclyl, optionally comprising one or more additional heteroatoms, such as 1 or 2 heteroatoms selected from oxygen, nitrogen or sulfur. In some embodiments, L¹ is —CH₂CH₂— or —CH₂CH₂CH₂—, but in other embodiments, L¹ and R¹ together with the nitrogen to which they are attached, form a 5- or 6-membered non-aromatic heterocyclyl ring, such as a piperidine or pyrrolidine ring. In some such embodiments, R² is C₁₋₆alkyl, such as C₁₋₄alkyl, C₁₋₃alkyl, or C₁₋₂alkyl, preferably methyl or ethyl.

R³ is aliphatic, cycloaliphatic, cycloalkylalkyl or alkoxyalkyl. In some embodiments, R³ is alkyl, such as C₁₋₆alkyl, C₁₋₄alkyl, C₁₋₃alkyl, or C₁₋₂alkyl; cycloalkyl, such as C₃₋₇cycloalkyl or C₃₋₄cycloalkyl; cycloalkylalkyl, such as —CH₂cycloalkyl; or alkoxyalkyl, such as C₁₋₆alkyl, C₁₋₄alkyl, C₁₋₃alkyl, or C₁₋₂alkyl substituted C₁₋₄alkoxy, C₁₋₂alkoxy, or C₃₋₆cycloalkyl. R³ may be linear or branched alkyl, and may be a linear C₁₋₆alkyl, C₁₋₄alkyl, C₁₋₃alkyl, or C₁₋₂alkyl or a branched C₃₋₆alkyl, C₃₋₄alkyl, or C₃alkyl. Exemplary linear and branched alkyl moieties include, but are not limited to, methyl, ethyl, n-propyl, or n-butyl, and isopropyl, isopropyl, tert-butyl, iso-butyl, or sec-butyl. In some embodiments, R³ is unsubstituted, but in other embodiments, R³ is substituted, and may be substituted with alkoxy, such as C₁₋₄alkoxy, or C₁₋₂alkoxy. Exemplary alkoxy substituents include, but are not limited to, methoxy, ethoxy, propoxy, isopropoxy or cycloalkoxy, such as cyclopropoxy.

Exemplary compounds within the scope of Formula I include:

Without being bound to a particular theory, when a compound according to Formula I disclosed herein is exposed to CO₂ it may exist in an equilibrium between Formulas II, III and IV, as shown below.

A person of ordinary skill in the art will understand that Formula II illustrates a zwitterionic state, while Formula IV illustrates the carbamic acid, and Formula III provides a potential intermediate state illustrating intramolecular bonding between the carbamate and the tertiary amine through the hydrogen. In some embodiments, when exposed to CO₂, the disclosed compounds form an equilibrium that comprises 50% or less of the zwitterionic form. The intramolecular H-bonding may shift the equilibrium toward a neutral form from the acid/base form. This in turn may reduce intermolecular bonding, which may help reduce the viscosity of the liquid, as the molecules can move more freely relative to each other. Additionally, limiting the compounds to a single hydrogen bond donor, such as a single secondary amine, also helps reduce the viscosity of the compound/CO₂ complex by reducing the intermolecular bonding between molecules.

Compounds according to Formula I may be prepared as exemplified below and as will be understood by a person of ordinary skill in the art or organic synthesis. With respect to the following exemplary reaction schemes, R¹, R², R³ and L¹ are as defined herein, and R⁴ is aliphatic, preferably alkyl, such as C₁₋₅alkyl, C₁₋₄alkyl, C₁₋₃alkyl, or C₁₋₂alkyl; cycloaliphatic, preferably cycloalkyl, such as C₃₋₇cycloalkyl or C₃₋₄cycloalkyl; or cycloalkylalkyl; or alkoxyalkyl, such that R⁴ and the carbon to which it is attached together form R³ as defined herein.

An exemplary synthesis may include the following first reaction step according to Scheme A.

Acid 1 activated by a suitable activating agent in a suitable solvent and treated with amine 2 to form amide 3. The activating agent may be any agent suitable to facilitate acid 1 coupling to amine 2. Suitable activating agents include, but are not limited to, boric acid; a carbodiimide reagent, such as 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC), or N,N′-dicyclohexylcarbodiimide (DCC), optionally in combination with hydroxybenzotriazole (HOBT); bis(2-oxo-3-oxazolidinyl)phosphinic chloride (BOP-Cl); thionyl chloride; mesyl chloride; tosyl chloride; or a combination thereof. Suitable solvents include, but are not limited to, aprotic solvents such as toluene, chlorinated solvents, such as chloroform or dichloromethane, dimethylformamide (DMF), tetrahydrofuran (THF), or a combination thereof. In some embodiments, the reaction may proceed with removal of water, such as by a drying agent or azeotropic water removal.

A second reaction step in the exemplary synthesis is provided below according to Scheme B.

Amine 3 is treated with reducing agent 4 to form diamine 5. Reducing agent 4 can be any suitable reducing agent, such as lithium aluminum hydride, borane-dimethylsulfide, borane-THF, or lithium borohydride. And the reaction may be performed in a suitable solvent, such as THF, methanol, ether, or a combination thereof. And the reaction maybe performed at a temperature suitable to facilitate the reaction, such as from room temperature to reflux of the solvent.

Alternatively, compounds according to Formula I may be made by the synthesis illustrated by Scheme C.

With respect to Scheme 3, R⁵ is H or alkyl, such as C₁-C₆alkyl, C₁₋₄alkyl, ethyl, or methyl. Carbonyl compound 6 is treated with amine 2 and a reducing agent to form diamine 7. Suitable reducing agents are known to persons of ordinary skill in the art and may include, but are not limited to, borohydride reagents, such as sodium borohydride, sodium cyanoborohydride, sodium triacetoxyborohydride, or lithium borohydride, or catalytic reduction, such as by hydrogen with a palladium, nickel, ruthenium, or platinum catalyst, such as palladium on carbon. The reaction may be performed in one step, such as by reductive amination, or in two steps, where the amine and carbonyl compound are first allowed to react to form an imine, before being contacted by the reducing agent. A person of ordinary skill in the art will understand which solvent(s) are suitable for the particular reaction being performed, but suitable solvents may include alcohols, such as methanol, ethanol, or isopropanol, toluene, THF, acetonitrile, or a combination thereof.

Another exemplary synthesis suitable to produce the compounds according to Formula I is provided by Scheme D.

Amine 2 is treated with compound 8 to form compound 4. Typically, amine 2 is provided in excess, such as 2×, 3×, 4×, 5× or more excess with respect to compound 8, to facilitate the formation of compound 4, both to drive the reaction to completion and to limit formation of a tertiary amine. X is a suitable leaving group, such as, halogen, for example, bromo or chloro, methylate, or tosylate. The reaction may be performed neat, i.e., without an additional solvent, or alternatively, the reaction may be performed using a solvent. And the reaction may be performed in the presence or absence of an additionally added base. Suitable solvents include, but are not limited to, chlorinated solvents, such as chloroform or dichloromethane, toluene, acetonitrile, DMF, THF, pyridine or a combination thereof. And suitable bases include any base that will facilitate the reaction, such as a trialkylamine, for example, trimethylamine, pyridine, or an inorganic base, such as potassium carbonate. In some embodiments, the reaction mixture is contacted with an aqueous base to remove excess amine, and/or neutralize any salt of the product that may have formed. The reaction may be performed at a temperature suitable to facilitate the reaction, such as from 20° C. to 120° C. or more, or to reflux of the reactants and/or solvent, such as from 30° C. to 100° C., from 40° C. to 80° C., or from 40° C. to 60° C. In some embodiments, the reaction temperature is selected such that the reaction is performed in a condensed phase.

III. Hydrogenation Catalyst

A hydrogenation catalyst is used in embodiments of the disclosed integrated processes. The catalyst may be heterogeneous or homogeneous hydrogenation catalyst. Suitable catalysts may include, but are not limited to, those comprising Cu, Zn, Al, Pd, Pt, Si, Cr, Ru, Rh, Co, Ni, Zr, Ti, Ce, M_(n), and combinations thereof. In some embodiments, the hydrogenation catalyst may comprise known materials to catalyze hydrogenation such (i) precious metals Pt, Pd, Rh, Ir, and Ru, base metals Ni, Co, Fe, Cu, and Zn, and catalysts supported on materials such as alumina, silica, carbon, and chromite and magnesium aluminum spinels, or any combination thereof; or (ii) a pincer ligand; or (iii) both (i) and (ii). Pincer ligands are often referred to as XYZ-type pincer ligands, where X, Y, and Z provide the identity of the coordinating atoms in the ligand. For example, a PNP pincer ligand chelates a metal atom via bonds with phosphorus, nitrogen, and phosphorus atoms of the ligand. Similarly, a PCP pincer ligand binds via phosphorus, carbon, and phosphorus. In certain embodiments, the pincer ligand is a NNP or PNP pincer. The pincer ligand backbone may be aromatic, heteroaromatic, aliphatic, or heteroaliphatic.

Suitable catalysts include, but are not limited to, Cu, ZnO, Al₂O₃, Pd, Pt, SiO₂, Cu₂Cr₂O₅, transition metal pincer-type catalysts (i.e., catalyst comprising one or more pincer ligands), and combinations thereof. In some embodiments, the transition metal pincer-type catalyst is a Ru-pincer catalyst. Two exemplary Ru-pincer catalysts are shown.

In some embodiments, the process is a conversion of CO₂ to methanol and water, and the catalyst comprises Cu, Zn, Al, or a combination thereof. In certain examples, the catalyst comprises Cu/ZnO/Al₂O₃.

In some embodiments, the process is a conversion of CO₂ to methanol and a glycol, and the catalyst is a Ru-based catalyst. In certain embodiments, the Ru-based catalyst is a Ru-pincer catalyst, such as

In some embodiments, the process is a conversion of CO₂ to methane, and the catalyst comprises Ru, Ni, Fe, Co, Rh, or a combination thereof. The metal may be disposed on a support. In some embodiments, the support is an oxide or carbon. Suitable oxide supports include, but are not limited to Al₂O₃, In₂O₃, CeO₂, TiO₂, SiO₂, ZrO₂, magnesium aluminum spinels (e.g., MgO—Al₂O₃), chromite (e.g., FeCr₂O₄), and combinations thereof. In certain examples, the catalyst comprises Ru/Al₂O₃ or Ru/C.

IV. Process for Producing Methanol

In some embodiments, an integrated process for capture and conversion of CO₂ to methanol includes combining the CO₂, a hydrogenation catalyst, and hydrogen with a condensed phase comprising an amine under conditions effective to form methanol and water. The methanol and water are separated from the amine, and subsequently from one another. The overall process is CO₂₊₃H₂→CH₃OH+H₂O, ΔH=−49.4 kJ/mol. In some embodiments, the reaction proceeds via formamide or formate ester intermediates in the condensed phase. In certain embodiments, after the methanol and water are separated, the methanol has a purity within a range of 99-99.9%, such as a purity of 99.5-99.8%.

In any of the foregoing or following embodiments, prior to combination with the CO₂, hydrogenation catalyst, and hydrogen, the condensed phase may comprise, consist of, or consist essentially of the amine. In one embodiment, the condensed phase consists of the amine. The condensed phase may consist of the amine when the amine is a liquid at ambient temperature or under the conditions at which the amine and CO₂ are combined. In an independent embodiment, the condensed phase consists essentially of the amine. In another independent embodiment, the condensed phase is a solution comprising the amine and a nonaqueous solvent. In still another embodiment, the condensed phase comprises the amine and from 0-10 wt % water, such as from 0-5 wt % water or 0-2 wt % water. In some examples, the condensed phase is substantially devoid of water prior to the conversion of CO₂ to methanol, e.g., the condensed phase prior to reaction may comprise less than 1 wt % water, prior to reaction.

In any of the foregoing or following embodiments, the condensed phase comprises an amine as previously discussed. In some embodiments, the condensed phase is a solution comprising a solvent and a tertiary amine. The solvent may be an alcohol. Suitable tertiary amines include, but are not limited to, trialkylamines. In some examples, the amine is triethylamine and the alcohol is an alkanol, e.g., ethanol. In certain working embodiments, the condensed phase comprised triethylamine:ethanol in a 1:10 molar ratio.

In any of the foregoing or following embodiments, the amine may be a compound according to Formula I, i.e., a CO₂BOL. In certain embodiments, the condensed phase consists of or consists essentially of the CO₂BOL. By “consists essentially of” or “consists of” is meant that no additional solvent is affirmatively added to the CO₂BOL. By “consists essentially of” is further meant that, prior to CO₂ capture and conversion, the condensed phase comprises at least 95 wt % of the CO₂BOL and comprises less than 1 wt % of any other amine. The condensed phase may be substantially devoid of water prior to the conversion of CO₂ to methanol, e.g., the condensed phase prior to reaction may comprise less than 5 wt % water, such as less than 1 wt % water, prior to reaction. The absence of a cosolvent in some embodiments maximizes the amine concentration and/or the concentration of captured CO₂, and concomitantly reduces the volume of the condensed phase and the energy required to heat the condensed phase to a desired temperature for CO₂ capture and subsequent conversion. The absence of a cosolvent also may simplify the downstream solvent separation and recovery unit.

In any of the foregoing or following embodiments, the hydrogenation catalyst may comprise known materials to catalyze hydrogenation such (i) precious metals Pt, Pd, Rh, Ir, and Ru, base metals Ni, Co, Fe, Cu, and Zn, and catalysts supported on materials such as alumina, silica, carbon, and chromite and magnesium aluminum spinels, or any combination thereof; or (ii) a pincer ligand; or (iii) both (i) and (ii). In some embodiments, the hydrogenation catalyst comprises a copper-containing catalyst, a zinc-containing catalyst, an aluminum-containing catalyst, or any combination thereof. In certain embodiments, the hydrogenation catalyst comprises Cu/ZnO/Al₂O₃.

FIG. 1 shows one exemplary embodiment of an integrated process for converting CO₂ to methanol. CO₂, a hydrogenation catalyst, hydrogen, and a condensed phase comprising an amine are combined (110). The combined components are subjected to an effective temperature T_(M) and initial pressure P_(M) for a sufficient time t_(M) to form a solution comprising CH₃OH, H₂O, and the condensed phase amine (120). In this step, carbon dioxide is captured by the amine in the condensed phase and is then hydrogenated to form methanol and water. By “captured” is meant that the CO₂ is adsorbed, absorbed, covalently bound (e.g., in the form of an alkylcarbonic acid or alkylcarbamic acid), or ionically bound to the amine in the condensed phase. In some embodiments, CO₂ capture is performed at a temperature ≤60° C., such as a temperature within a range of 25-60° C. In certain embodiments where T_(M)>60° C., a period of time is allowed for CO₂ capture before increasing the temperature to T_(M). A separation is performed to provide a solution comprising CH₃OH and H₂O (130) and the condensed phase amine (140), which optionally is recycled and reused. The separation may be performed by any suitable method. In some embodiments, a flash separation or distillation may be performed. For example, a flash drum or distillation column may be used to separate methanol and water vapor from the condensed phase amine. In another example, a distillation column may be used when the condensed phase amine has a boiling point close to that of methanol or water. The CH₃OH and H₂O are then separated to provide CH₃OH (150) and H₂O (160). In some embodiments, the CH₃OH and H₂O are separated by distillation.

In contrast to other methods utilizing gas-phase CO₂ hydrogenation, embodiments of the disclosed process provide CO₂ hydrogenation in the condensed phase. Additionally, no mechanical compression of the CO₂ is needed for transport. Advantageously, the CO₂ hydrogenation process is exothermic, which partially offsets the energy required for CO₂ capture and subsequent separation of the methanol and water vapor from the condensed phase amine. Moreover, in some embodiments, the amine synergistically acts as a co-catalyst for the hydrogenation reaction by charge stabilizing the captured CO₂ in the form of charged/polar intermediates as discussed in more detail below. In some embodiments, the disclosed process reduces capital and operating costs by at least 20% relative to a benchmark methanol synthesis via gas-phase CO₂ hydrogenation.

In any of the foregoing or following embodiments, the temperature T_(M) may be within a range of from 50° C. to 180° C., such as from 100° C. to 180° C., 110-170° C., 120-150° C., or 120-130° C. In any of the foregoing embodiments, the initial pressure P_(M) may be within a range of from 1 MPa to 10 MPa. In some embodiments, the initial pressure is from 1-6 MPa, 2-5 MPa, or 2-4 MPa. In contrast, conventional syngas conversion technologies are performed at temperatures ranging from 250-300° C. and pressures ranging from 5-10 MPa. Advantageously, in some embodiments, the lower temperature of the disclosed process favors the exothermic formation of CH₃OH over the endothermic reverse water gas shift reaction (CO₂+H₂→CO+H₂O, ΔH=+41.1 kJ/mol), thereby increasing selectivity for methanol formation. In any of the foregoing or following embodiments, the time t_(M) may be within a range of from 3 seconds to 36 hours, such as from 15 seconds to 36 hours, 30 seconds to 36 hours, 1 minute to 36 hours, 5 minutes to 36 hours, 10 minutes to 36 hours, 30 minutes to 36 hours, 1-36 hours, 3-36, 6-24 hours, 12-18 hours, 3 seconds to 3 hours, 3 seconds to 1 hour, 5 seconds to 1 hour, 10 seconds to 30 minutes, or 15 seconds to 15 minutes. In a continuous process, the time t_(M) may be on the order of a few seconds to several minutes, whereas the time t_(M) may be on the order of several hours in a batch process. As set forth above, in certain embodiments where T_(M)>60° C., a period of time is allowed for CO₂ capture before increasing the temperature to T_(M). The period of time for initial CO₂ capture may range from a few seconds to several hours, with longer times being used in batch processes.

In some embodiments, the conversion of CO₂ to methanol is a one-pot, one-step process as shown in FIG. 1, where CO₂(g) and H₂(g) are combined with the hydrogenation catalyst and a condensed phase amine in a vessel and subjected to effective conditions for the reaction to occur. The hydrogenation catalyst may be disposed in a fixed bed or a fluidized bed.

In any of the foregoing embodiments, the condensed phase amine may be an amine-containing solvent, a solution comprising the amine and a solvent, or a solid state (e.g., an amine supported on SiO₂).

In an independent embodiment, the CO₂ is present in the condensed phase with the amine prior to combination with hydrogen and the hydrogenation catalyst. In certain examples, the conversion of CO₂ to methanol is a two-step process. As shown in FIG. 2, CO₂ is combined with a condensed phase amine (202) under effective conditions to form a condensed phase comprising the amine, the CO₂, and optionally a solvent (204). The effective conditions include a temperature T′, an initial pressure P′, and time t′ sufficient for the condensed phase amine to capture CO₂, thereby providing a condensed phase comprising the amine and captured CO₂. In other embodiments, the process may commence with a condensed phase comprising the amine and captured CO₂ (204). The condensed phase comprising the amine and CO₂ is combined with H₂ and the hydrogenation catalyst (210). The combined components are subjected to an effective temperature T_(M) and initial pressure P_(M) for a sufficient time t_(M) to form a solution comprising CH₃OH, H₂O, and the condensed phase amine (220), where T_(M), P_(M), and t_(M) are as described above. A separation is performed to provide a solution comprising CH₃OH and H₂O (230) and the condensed phase amine (240), which optionally is recycled and reused. The separation may be performed by any suitable method as set forth above. The CH₃OH and H₂O are then separated to provide CH₃OH (250) and H₂O (260). In some embodiments, the CH₃OH and H₂O are separated by distillation.

In any of the foregoing or following embodiments, the temperature T′ may be within a range of from 20° C. to 60° C., such as from 25-50° C. In some embodiments, T′ is ambient temperature, e.g., 20-30° C. In any of the foregoing embodiments, the initial pressure P′ may be within a range of from 0.1 MPa to 5 MPa, such as an initial pressure within a range of from 0.1-2 MPa or 0.1-1 MPa. In any of the foregoing embodiments, the CO₂ capture may be substantially immediate. In some embodiments, the time t′ is within a range of from greater than zero seconds to 36 hours, such as from 3 seconds to 36 hours, 15 seconds to 36 hours, 30 seconds to 36 hours, 1 minute to 36 hours, 5 minutes to 36 hours, 10 minutes to 36 hours, 30 minutes to 36 hours, 1-36 hours, 3-36, 6-24 hours, 12-18 hours, 3 seconds to 3 hours, 3 seconds to 1 hour, 5 seconds to 1 hour, 10 seconds to 30 minutes, or 15 seconds to 15 minutes. In a continuous process, the time t′ may be on the order of a few seconds to several minutes, whereas the time t′ may be on the order of several hours in a batch process.

In any of the foregoing or following embodiments, the amine may be a tertiary amine or a diamine (e.g., a compound according to Formula I), and the amine and CO₂ may be combined in a molar ratio of from 1:4 to 10:1, such as from 1:2 to 5:1 or 1:1 to 5:1. Alternatively, the amine may be a polyamine, and the polyamine and CO₂ may be combined in a molar ratio of from 1:25 to 1:300, such as from 1:50 to 1:150.

In any of the foregoing or following embodiments, the H₂ may be in stoichiometric excess relative to the CO₂. Excess H₂ shifts the reaction equilibrium and increases CO₂ to methanol conversion. The stoichiometric H₂:CO₂ ratio for reaction in the condensed phase is 3:1. In some embodiments, the amine present in the condensed phase is the limiting reagent such that not all of the CO₂ is captured by the amine, and the amount of H₂ added is in stoichiometric excess relative to amount of CO₂ in the condensed phase, e.g., CO₂ captured by the amine in the condensed phase. In certain embodiments, the process is performed with a molar ratio of added H₂ to CO₂ from 2:1 to 10:1, such as from 4:1 to 6:1, providing a molar ratio of H₂ to CO₂ in the condensed phase of from greater than 3:1 to 10:1. Excess H₂ may be recycled as shown in FIGS. 1 and 2.

In any of the foregoing or following embodiments, CO₂ captured by the amine may be the limiting reactant, and at least 25 mol %, at least 50 mol %, at least 75 mol %, at least 90 mol %, or even at least 95 mol % of the captured CO₂ is consumed. For example, from 25-100, 50-100, 75-100, 90-100, or 95-100 mol % of the captured CO₂ may be consumed. Alternatively, at least 10 mol %, at least 25 mol %, at least 50 mol %, or at least 75 mol % of CO₂ combined with the hydrogen, hydrogen catalyst, and condensed phase comprising the amine is consumed. For example, from 10-100, 25-100, 50-100, or 75-100 mol % of the CO₂ may be consumed.

FIG. 3A is a schematic diagram of one embodiment of an apparatus 300 for performing the method of FIG. 2. A condensed phase comprising an amine and a gas flow comprising CO₂ are flowed into a CO₂ absorber 310. The condensed phase comprising the amine and the CO₂ are subjected to a temperature T′, an initial pressure P′, and time t′ sufficient for the amine to capture the CO₂ to form a condensed phase solution comprising the amine and CO₂. A CO₂-depleted gas is exhausted from the CO₂ absorber 310. The condensed phase solution flows into a hydrogenation reactor 320 where it is combined with H₂ and a hydrogenation catalyst disposed within the hydrogenation reactor 320. In any of the foregoing embodiments, the hydrogenation catalyst may be disposed in a fixed bed or a fluidized bed within the hydrogenation reactor 320. If the bed is a fluidized bed, the H₂ flow and/or the condensed phase solution may be used for the fluidization. The combined components are subjected to an effective temperature T_(M) and initial pressure P_(M) for a sufficient time t_(M) to form a condensed phase solution comprising CH₃OH, H₂O, and the amine. In any of the foregoing embodiments, the condensed phase solution, the hydrogen, or both may be pre-heated to a temperature ranging from greater than ambient to T_(M) before entering the hydrogenation reactor. In some embodiments, the H₂ flowing into the hydrogenation reactor 320 is in stoichiometric excess relative to the CO₂ in the condensed phase, and excess H₂ exiting the hydrogenation reactor optionally is recycled back into the hydrogenation reactor. Excess H₂ shifts the reaction equilibrium and increases CO₂ to methanol conversion. In certain embodiments, e.g., as shown in FIG. 3B, the excess H₂ and the condensed phase solution comprising the CH₃OH, H₂O, and amine exiting the hydrogenation reactor 320 are flowed into a separator 325 where the H₂ is separated from the solution and subsequently recycled to the hydrogenation reactor 320. The H₂ and the condensed phase solution may be cooled, e.g., to 30-50° C. before entering the separator 325. In some embodiments, the separator 325 is a flash drum, such as a low-temperature flash drum operating at a temperature of 20-50° C., such as 30-50° C., and a pressure of 1-5 MPa, such as 2-3 MPa. The condensed phase solution comprising the CH₃OH, H₂O, and the amine exiting the hydrogenation reactor 320 (or separator 325) is flowed into a separator 330 where the condensed phase amine is separated from the CH₃OH and H₂O. In some embodiments, the separator 330 is a flash drum or a distillation column. In certain embodiments, the separator 330 is a flash drum operating at a temperature within a range of from 100-150° C., such as 120-140° C., and a pressure of 0.1-1 MPa, such as 0.1-0.5 MPa. The condensed phase amine optionally is recycled back to the CO₂ absorber 310. The CH₃OH and H₂O are flowed into a distillation column 340 where the CH₃OH and H₂O are separated.

In any of the foregoing embodiments, the process may be a batch process or a continuous process. Some embodiments of the apparatus shown in FIGS. 3A and 3B may be suitable for a continuous process. In a continuous process, the hydrogenation catalyst may be disposed in a fixed bed in the hydrogenation reactor 320, and both H₂ and the condensed phase solution comprising the amine and CO₂ flow through or across the fixed bed. Alternatively, the hydrogenation catalyst may be disposed in a fluidized bed in the hydrogenation reactor 320, where the bed is fluidized by the H₂ flow and/or the condensed phase solution, and the H₂ and the condensed phase solution comprising the amine and CO₂ flow through the fluidized bed. In some embodiments, the condensed phase solution is flowed through the hydrogenation reactor at a space velocity of from greater than 0 to 50,000 hr⁻¹, such as from 1-50,000, 100-50,000, 500-40,000, 500-30,000, or 1,000-25,000 hr⁻¹. In some embodiments, a lower space velocity provides an increased conversion of CO₂ to methanol.

In an exemplary embodiment, CO₂ is combined with a condensed phase comprising triethylamine and ethanol (1:10 molar ratio, e.g., 2 mmol triethylamine and 20 mmol ethanol), a hydrogenation catalyst (e.g., Cu/ZnO/Al₂O₃) and excess hydrogen relative to an amount of the CO₂ captured by the amine in the condensed phase. The combined components are heated to 170° C. for 16-24 h, thereby converting CO₂ to methanol and water.

V. Process for Coproducing Methanol and a Glycol

Industrially, methanol is produced from a syngas mixture in the presence of a Cu-based catalyst at high temperature and high pressure. The reaction byproduct, water, is separated from the methanol by distillation and is not further utilized, which reduces the net atom efficiency. Glycols typically are produced by hydrolysis of epoxides in excess water under acidic conditions at high temperatures (>150° C.). In addition, the hydrolysis product stream is often contaminated with glycol oligomers, resulting in an energy penalty associated with separating glycol(s) from water and the oligomers.

Disclosed herein is an integrated process for capture and conversion of CO₂ to methanol and a glycol that concurrently produces a 1:1 mixture of methanol and the glycol. Advantageously, the process does not require addition of water. Instead, water is formed in situ and immediately consumed. In some embodiments, the disclosed process includes combining an epoxide, a hydrogenation catalyst, hydrogen, and CO₂ with a condensed phase comprising an amine under conditions effective to form methanol and a glycol. In any of the foregoing or following embodiments, the reactants may consist of, or consist essentially of, the epoxide, the hydrogenation catalyst, hydrogen, CO₂, and the condensed phase. In such embodiments, water is not added as a reactant. In this context, “consists essentially of” means that the reactants include no more than trace amounts (e.g., less than 0.5 wt %) water and other components that may participate in the reaction are excluded. Overall, the net reaction is:

where R and R′ independently are H or aliphatic, or R and R′ together with the carbon atoms to which they are attached form a cycloaliphatic ring. By varying R and R′ on the epoxide, different glycols can be made. In some embodiments, R and R′ independently are H or C₁-C₁₀ alkyl. In an independent embodiment, R is H or C₁-C₁₀ alkyl and R′ is H. In certain embodiments, R is H, methyl, or ethyl, and R′ is H. In particular examples, R is methyl and R′ is H such that the epoxide is propylene oxide and the resulting glycol is propylene glycol. In another independent embodiment, R and R′ together with the carbon atoms to which they are attached form a cycloaliphatic ring. In one non-limiting example, the epoxide is cyclohexene oxide (also known as 1,2-epoxycyclohexane or 7-oxabicyclo[4.1.0]heptane) and the resulting glycol is cyclohexane-1,2-diol:

By combining both syntheses in a single process, the energy intensive separations associated with the individual processes are avoided. Instead, the byproduct from one reaction is the feed for another reaction. Additionally, the unique reactivity of the CO₂ captured by the amine (i.e., the nucleophilicity of the anionic “captured” CO₂ as an alkylcarbonate or alkylcarbamate) is exploited to open the epoxide ring, forming a cyclic carbonate, which is then hydrogenated to produce methanol and glycol at the same time. In some embodiments, the process exhibits 100% atom efficiency with no waste.

In any of the foregoing or following embodiments, prior to combination with the CO₂, epoxide, hydrogenation catalyst, and hydrogen, the condensed phase may comprise, consist of, or consist essentially of the amine. In one embodiment, the condensed phase consists of the amine. The condensed phase may consist of the amine when the amine is a liquid at ambient temperature or under the conditions at which the amine and CO₂ are combined. In an independent embodiment, the condensed phase consists essentially of the amine. In another independent embodiment, the condensed phase is a solution comprising the amine and a nonaqueous solvent. In still another embodiment, the condensed phase comprises the amine and from 0-10 wt % water, such as from 0-5 wt % water or 0-2 wt % water. In some examples, the condensed phase is substantially devoid of water prior to the conversion of CO₂ to methanol, e.g., the condensed phase prior to reaction may comprise less than 1 wt % water, prior to reaction.

The condensed phase comprises an amine. In some embodiments, the amine is a polyamine or a compound according to Formula I as previously discussed. In certain embodiments, the amine is a polyamine, such as polyethyleneimine (PEI). In some examples, the amine is a branched PEI with a molecular weight, M_(n), within a range of 500-2,000 g/mole. In some embodiments, the condensed phase comprises PEI and a nonaqueous solvent. In certain examples, the amine is branched PEI and the nonaqueous solvent is tetrahydrofuran. In an independent embodiment, the condensed phase consists of, or consists essentially of, a compound according to Formula I.

In any of the foregoing or following embodiments, the amine may be a polyamine, and the polyamine and CO₂ may be combined in a molar ratio of from 1:5 to 1:300, such as from 1:25 or 1:250 or 1:50 to 1:150. Alternatively, the amine may be a tertiary amine or a diamine (e.g., a compound according to Formula I), and the amine and CO₂ may be combined in a molar ratio of from 1:4 to 10:1, such as from 1:2 to 5:1 or 1:1 to 5:1.

In any of the foregoing or following embodiments, the hydrogenation catalyst may comprise known materials to catalyze hydrogenation such (i) precious metals Pt, Pd, Rh, Ir, and Ru, base metals Ni, Co, Fe, Cu, and Zn, and supported on materials such as alumina, silica, carbon, and chromite and magnesium aluminum spinels, or any combination thereof; or (ii) a pincer ligand; or (iii) both (i) and (ii). In some embodiments, the hydrogenation catalyst comprises a ruthenium-based catalyst, such as a ruthenium-pincer catalyst. In certain embodiments, the hydrogenation catalyst is:

In any of the foregoing or following embodiments, the stoichiometric H₂:CO₂ ratio for reaction in the condensed phase is 3:1. In some embodiments, the amine present in the condensed phase is the limiting reagent such that not all of the CO₂ is captured by the amine, and the amount of H₂ added is in stoichiometric excess relative to amount of CO₂ in the condensed phase, e.g., CO₂ captured by the amine in the condensed phase. In certain embodiments and the amount of H₂ added is in stoichiometric excess relative to amount of CO₂ in the condensed phase, e.g., CO₂ captured by the amine in the condensed phase. In some embodiments, the process is performed with a molar ratio of added H₂ to CO₂ from 2:1 to 10:1, such as from 4:1 to 6:1, providing a molar ratio of H₂ to CO₂ in the condensed phase of from greater than 3:1 to 10:1.

In any of the foregoing or following embodiments, at least 10 mol % of the epoxide may be consumed in the reaction. In some embodiments, the epoxide is a limiting reactant and at least 25 mol %, at least 50 mol %, at least 75 mol %, at least 90 mol %, or even at least 95 mol % of the epoxide is consumed. For example, from 25-100, 50-100, 75-100, 90-100, or 95-100 mol % of the epoxide may be consumed.

Alternatively, CO₂ captured by the amine may be the limiting reactant, and at least 25 mol %, at least 50 mol %, at least 75 mol %, at least 90 mol %, or even at least 95 mol % of the captured CO₂ is consumed. For example, from 25-100, 50-100, 75-100, 90-100, or 95-100 mol % of the captured CO₂ may be consumed. In another alternative, at least 10 mol %, at least 25 mol %, at least 50 mol %, or at least 75 mol % of CO₂ combined with the epoxide, hydrogen, hydrogen catalyst, and condensed phase comprising the amine is consumed. For example, from 10-100, 25-100, 50-100, or 75-100 mol % of the CO₂ may be consumed.

FIG. 4 shows one exemplary embodiment of the process. The CO₂, an epoxide, a hydrogenation catalyst, and hydrogen are combined with a condensed phase comprising an amine and optionally a solvent (410). The combined components are subjected to an effective temperature T_(MG) and initial pressure P_(MG) for a sufficient time t_(MG) to form a solution comprising CH₃OH, glycol, and the condensed phase comprising the amine (420). One possible mechanism for the reaction is shown in exemplary Scheme 1 below where the epoxide is propylene oxide PO. Carbon dioxide is captured by the amine in the condensed phase and reacts with the epoxide to form a cyclic carbonate (propylene carbonate PC, Step (a)). The cyclic carbonate is hydrogenated and proceeds through Intermediates A-C to form the glycol and formamide (Steps i-iv). Steps ii and iv are equilibria. The formamide obtained in Step iv is further hydrogenated to form methanol (Step v).

A separation is performed to provide (i) CH₃OH (430) and (ii) a solution comprising glycol and the condensed phase comprising the amine (440). The separation may be performed by any suitable method. In some embodiments, the solution is cooled, e.g., to a temperature within a range of from −100° C. to 30° C. prior to separating the methanol from the condensed phase. In certain embodiments, a flash and/or distillation separation may be performed. For example, a flash drum may be used to separate methanol from the glycol and the condensed phase comprising the amine. The solution comprising glycol and the condensed phase 440 may be separated by distillation to provide the condensed phase comprising the amine (450) and the glycol (460). The condensed phase comprising the amine (450) is optionally recycled and reused. In some examples, the CH₃OH and glycol may be removed together from the amine, and the CH₃OH and glycol are then separated, e.g., by distillation (not shown).

In any of the foregoing embodiments, the temperature T_(MG) may be within a range of from 50° C. to 170° C., such as from 100° C. to 170° C., 120-160° C. or 130-150° C. In any of the foregoing embodiments, the initial pressure P_(MG) may be within a range of from 3 MPa to 10 MPa. In some embodiments, the initial pressure is from 3-7 MPa or 5-7 MPa. In any of the foregoing embodiments, the time t_(MG) may be within a range of from 3 seconds to 36 hours, such as from 15 seconds to 36 hours, 30 seconds to 36 hours, 1 minute to 36 hours, 5 minutes to 36 hours, 10 minutes to 36 hours, 30 minutes to 36 hours, 1-36 hours, 3-36, 6-24 hours, 12-18 hours, 3 seconds to 3 hours, 3 seconds to 1 hour, 5 seconds to 1 hour, 10 seconds to 30 minutes, or 15 seconds to 15 minutes. In a continuous process, the time t_(MG) may be on the order of a few seconds to several minutes, whereas the time t_(MG) may be on the order of several hours in a batch process. In certain embodiments where T_(MG)>60° C., a period of time is allowed for CO₂ capture before increasing the temperature to T_(MG). The period of time for initial CO₂ capture may range from a few seconds to several hours, with longer times being used in batch processes.

In an independent embodiment, the CO₂ is present in the condensed phase comprising the amine prior to combining the condensed phase with the epoxide, hydrogenation catalyst, and hydrogen. In such embodiments, the condensed phase is a solution comprising the amine, CO₂, and optionally a solvent. As shown in FIG. 5, the process optionally includes combining CO₂ with a condensed phase comprising the amine (502) under effective conditions to form a condensed phase comprising the amine, the CO₂, and optionally a solvent (504). The effective conditions include a temperature T′, an initial pressure P′, and time t′ sufficient for the condensed phase amine to capture CO₂, thereby providing a condensed phase comprising the amine and captured CO₂. The temperature T′, initial pressure P′, and time t′ are as previously described. In other embodiments, the process may commence with a condensed phase comprising the amine and captured CO₂ (504). The condensed phase comprising the amine and CO₂ is combined with the epoxide, the hydrogenation catalyst, and hydrogen (510). The combined components are subjected to an effective temperature T_(MG) and initial pressure P_(MG) for a sufficient time t_(MG) to form a solution comprising CH₃OH, glycol, and the condensed phase comprising the amine (520), where T_(MG), P_(MG), and t_(MG) are as described above. A separation is performed to provide CH₃OH (530) and a solution comprising glycol and the condensed phase comprising the amine (540). The separation may be performed by any suitable method. In some embodiments, a flash separation or distillation may be performed by any suitable method as set forth above. The glycol and the condensed phase comprising the amine are then separated (e.g., by distillation) to provide the condensed phase comprising the amine (550) and glycol (560). The condensed phase comprising the amine (550) is optionally recycled and reused. In some examples, the CH₃OH and glycol may be removed together from the amine, and the CH₃OH and glycol are then separated, e.g., by distillation (not shown).

In some embodiments, the process of converting the captured CO₂ to methanol and glycol is a two-step process, comprising (i) cycloaddition of the CO₂ to the epoxide to form a cyclic carbonate-containing mixture comprising a cyclic carbonate and the condensed phase amine and (ii) subsequent hydrogenation of the cyclic carbonate-containing mixture in the presence of a hydrogenation catalyst and H₂ to form methanol and glycol. Embodiments of the disclosed process do not require added water. Instead, oxygen and hydrogen are produced and consumed in situ. FIG. 6 illustrates one embodiment of a two-step process. CO₂ is combined with an epoxide and a condensed phase comprising an amine (606). The combined components are subjected to an effective temperature T_(C) and initial pressure P_(C) for a sufficient time t_(C) to form a cyclic carbonate-containing mixture comprising a cyclic carbonate and the condensed phase comprising the amine (608). The cyclic carbonate-containing mixture is combined with a hydrogenation catalyst and H₂ (610) and subjected to an effective temperature T_(MG) and initial pressure P_(MG) for a sufficient time t_(MG) to form a solution comprising CH₃OH, glycol, and the condensed phase comprising the amine (620), where T_(MG), P_(MG), and t_(MG) are as previously described. In an alternative embodiment, CO₂ is combined with an epoxide, a condensed phase comprising an amine, and a hydrogenation catalyst (607). The combined components are subjected to an effective temperature T_(C) and initial pressure P_(C) for a sufficient time t_(C) to form a cyclic carbonate-containing mixture comprising a cyclic carbonate and the amine (609). The cyclic carbonate-containing mixture, along with the hydrogenation catalyst, is combined with H₂ (610) and subjected to an effective temperature T_(MG) and initial pressure P_(MG) for a sufficient time t_(MG) to form a solution comprising CH₃OH, glycol, and the condensed phase comprising the amine (620), where T_(MG), P_(MG), and t_(MG) are as previously described. In any of the foregoing embodiments, a separation then is performed to provide CH₃OH (630) and a solution comprising the glycol and the condensed phase comprising the amine (640). The separation may be performed by any suitable method. In some embodiments, a flash separation or distillation may be performed by any suitable method as set forth above. The glycol and the condensed phase comprising the amine are then separated (e.g., by distillation) to provide the condensed phase comprising the amine (650) and glycol (660). The condensed phase comprising the amine (650) is optionally recycled and reused. In some examples, the CH₃OH and glycol may be removed together from the amine, and the CH₃OH and glycol are then separated, e.g., by distillation (not shown).

In any of the foregoing embodiments, the temperature T_(C) may be within a range of from 25° C. to 180° C., such as from 100-180° C., 100-170° C., 120-160° C., or 130-150° C. In any of the foregoing embodiments, the initial pressure P_(C) may be within a range of from 0.1 MPa to 2 MPa, such as 0.1-1 MPa. In any of the foregoing embodiments, the time t_(C) may be within a range of from 3 seconds to 36 hours, such as from 15 seconds to 36 hours, 30 seconds to 36 hours, 1 minute to 36 hours, 5 minutes to 36 hours, 10 minutes to 36 hours, 30 minutes to 36 hours, 1-36 hours, 3-36, 6-24 hours, 12-18 hours, 3 seconds to 3 hours, 3 seconds to 1 hour, 5 seconds to 1 hour, 10 seconds to 30 minutes, or 15 seconds to 15 minutes. In a continuous process, the time to may be on the order of a few seconds to several minutes, whereas the time to may be on the order of several hours in a batch process. In certain embodiments where T_(C)>60° C., a period of time is allowed for CO₂ capture before increasing the temperature to T_(C). The period of time for initial CO₂ capture may range from a few seconds to several hours, with longer times being used in batch processes.

In an independent embodiment, the CO₂ is present in the condensed phase comprising the amine prior to combining the condensed phase with the epoxide, hydrogenation catalyst, and hydrogen. In such embodiments, the condensed phase is a solution comprising the amine, CO₂, and optionally a solvent. As shown in FIG. 7, the process optionally includes combining CO₂ with a condensed phase comprising the amine (702) under effective conditions to form a condensed phase comprising the amine, the CO₂, and optionally a solvent (704). The effective conditions include a temperature T′, an initial pressure P′, and time t′ sufficient for the condensed phase amine to capture CO₂, thereby providing a condensed phase comprising the amine and captured CO₂. The temperature T′, initial pressure P′, and time t′ are as previously described. In other embodiments, the process may commence with a condensed phase comprising the amine and captured CO₂ (704). The condensed phase comprising the amine and CO₂ is combined with the epoxide (706). The combined components are subjected to an effective temperature T_(C) and initial pressure P_(C) for a sufficient time t_(C) to form a cyclic carbonate-containing mixture comprising a cyclic carbonate and the condensed phase comprising the amine (708). T_(C), P_(C), and t_(C) are as previously described. The cyclic carbonate-containing mixture is combined with a hydrogenation catalyst and H₂ (710) and subjected to an effective temperature T_(MG) and initial pressure P_(MG) for a sufficient time t_(MG) to form a solution comprising CH₃OH, glycol, and the condensed phase comprising the amine (720), where T_(MG), P_(MG), and t_(MG) are as previously described. In an alternative embodiment, CO₂ is combined with an epoxide, a condensed phase comprising an amine, and a hydrogenation catalyst (707). The combined components are subjected to an effective temperature T_(C) and initial pressure P_(C) for a sufficient time t_(C) to form a cyclic carbonate-containing mixture comprising a cyclic carbonate and the amine (709). The cyclic carbonate-containing mixture, along with the hydrogenation catalyst, is combined with H₂ (710) and subjected to an effective temperature T_(MG) and initial pressure P_(MG) for a sufficient time t_(MG) to form a solution comprising CH₃OH, glycol, and the condensed phase comprising the amine (720), where T_(MG), P_(MG), and t_(MG) are as previously described. In any of the foregoing embodiments, a separation then is performed to CH₃OH (730) and a solution comprising the glycol and the condensed phase comprising the amine (740). The separation may be performed by any suitable method. In some embodiments, a flash separation or distillation may be performed by any suitable method as set forth above. The glycol and the condensed phase comprising the amine are then separated (e.g., by distillation) to provide the condensed phase comprising the amine (750) and glycol (760). The condensed phase comprising the amine (750) is optionally recycled and reused. In some examples, the CH₃OH and glycol may be removed together from the amine, and the CH₃OH and glycol are then separated, e.g., by distillation (not shown).

FIG. 8A is a schematic diagram of one embodiment of an apparatus 800 for performing the method of FIG. 5. A condensed phase comprising an amine and a gas flow comprising CO₂ are flowed into a CO₂ absorber 810. The condensed phase and the CO₂ are subjected to a temperature T′, an initial pressure P′, and time t′ sufficient for the amine to capture the CO₂ to form a condensed phase solution comprising the amine and CO₂. A CO₂-depleted gas is exhausted from the CO₂ absorber 810. The condensed phase solution flows into a hydrogenation reactor 820 where it is combined with H₂, an epoxide, and a hydrogenation catalyst disposed within the hydrogenation reactor 820. In any of the foregoing embodiments, the hydrogenation catalyst may be disposed in a fixed bed or a fluidized bed within the hydrogenation reactor 820. If the bed is a fluidized bed, the H₂ flow may be used for the fluidization. The combined components are subjected to an effective temperature T_(MG) and initial pressure P_(MG) for a sufficient time t_(MG) to form a condensed phase solution comprising CH₃OH, glycol, and the amine. T_(MG), P_(MG), and t_(MG) are as previously described. In any of the foregoing embodiments, the condensed phase solution, the hydrogen, or both may be pre-heated to a temperature ranging from greater than ambient to T_(MG) before entering the hydrogenation reactor. In some embodiments, the H₂ flowing into the hydrogenation reactor 820 is in stoichiometric excess relative to the CO₂ in the condensed phase, and excess H₂ exiting the hydrogenation reactor optionally is recycled back into the hydrogenation reactor. In certain embodiments, e.g., as shown in FIG. 8B, the excess H₂ and the condensed phase solution comprising the CH₃OH, glycol, and the amine exiting the hydrogenation reactor 820 are flowed into a separator 825 where the H₂ is separated from the solution and subsequently recycled to the hydrogenation reactor 820. The H₂ and the condensed phase solution may be cooled, e.g., to 30-50° C. before entering the separator 825. In some embodiments, the separator 825 is a flash drum, such as a low-temperature flash drum operating at a temperature of 20-50° C., such as 30-50° C., and a pressure of 1-5 MPa, such as 2-3 MPa. The condensed phase solution comprising the CH₃OH, glycol, and the amine exiting the hydrogenation reactor 820 (or separator 825) is flowed into a separator 830 where the glycol and condensed phase amine are separated from the CH₃OH. In some embodiments, the separator 830 is a flash drum or a distillation column. In certain embodiments, the separator 830 is a flash drum operating at a temperature within a range of from 100-150° C., such as 120-140° C., and a pressure of 0.1-1 MPa, such as 0.1-0.5 MPa. The glycol and condensed phase amine are flowed into a distillation column 840 where the glycol and the condensed phase amine are separated. The condensed phase amine optionally is recycled back to the CO₂ absorber 810.

In an alternative embodiment, the apparatus 800 of FIGS. 8A and 8B may be used to perform the method of FIG. 7. In such an embodiment, the condensed phase solution flows into a hydrogenation reactor 820 where it is combined with the epoxide and the hydrogenation catalyst disposed within the hydrogenation reactor 820. The combined components are subjected to an effective temperature T_(C) and initial pressure P_(C) for a sufficient time t_(C) to form a cyclic carbonate-containing mixture comprising a cyclic carbonate and the amine. T_(C), P_(C), and t_(C) are as previously described. H₂ is then flowed into the hydrogenation reactor 820, and the combined components are subjected to an effective temperature T_(MG) and initial pressure P_(MG) for a sufficient time t_(MG) to form a condensed phase solution comprising CH₃OH, glycol, and the amine. T_(MG), P_(MG), and t_(MG) are as previously described. The remainder of the process proceeds as described above.

FIG. 9A is a schematic diagram of one embodiment of an apparatus 900 for performing the method of FIG. 7. A condensed phase amine and a gas flow comprising CO₂ are flowed into a CO₂ absorber 910. The condensed phase amine and CO₂ are subjected to a temperature T′, an initial pressure P′, and time t′ sufficient for the condensed phase amine to capture the CO₂ to form a condensed phase solution comprising the amine and CO₂. A CO₂-depleted gas is exhausted from the CO₂ absorber 910. The condensed phase solution flows into a cycloaddition reactor 915 where it is combined with an epoxide. The combined components are subjected to an effective temperature T_(C) and initial pressure P_(C) for a sufficient time t_(C) to form a cyclic carbonate-containing mixture comprising a cyclic carbonate and the amine. T_(C), P_(C), and t_(C) are as previously described. The cyclic carbonate-containing mixture is flowed into the condensed phase hydrogenation reactor 920 where it is combined with H₂ and a hydrogenation catalyst disposed within the hydrogenation reactor 920. In any of the foregoing embodiments, the hydrogenation catalyst may be disposed in a fixed bed or a fluidized bed within the hydrogenation reactor 920. If the bed is a fluidized bed, the H₂ flow may be used for the fluidization. The combined components are subjected to an effective temperature T_(MG) and initial pressure P_(MG) for a sufficient time t_(MG) to form a condensed phase solution comprising CH₃OH, glycol, and the amine. In any of the foregoing embodiments, the cyclic carbonate-containing mixture, the hydrogen, or both may be pre-heated to a temperature ranging from greater than ambient to T_(MG) before entering the hydrogenation reactor. In some embodiments, the H₂ flowing into the hydrogenation reactor 920 is in stoichiometric excess relative to the CO₂ in the condensed phase, and excess H₂ exiting the hydrogenation reactor optionally is recycled back into the hydrogenation reactor. In certain embodiments, e.g., as shown in FIG. 9B, the excess H₂ and the condensed phase solution comprising the CH₃OH, glycol, and amine exiting the hydrogenation reactor 920 are flowed into a separator 925 where the H₂ is separated from the solution and subsequently recycled to the hydrogenation reactor 920. The H₂ and the solution may be cooled, e.g., to 30-50° C. before entering the separator 925. In some embodiments, the separator 925 is a flash drum, such as a low-temperature flash drum operating at a temperature of 20-50° C., such as 30-50° C., and a pressure of 1-5 MPa, such as 2-3 MPa. The condensed phase solution comprising the CH₃OH, glycol, and amine exiting the hydrogenation reactor 920 (or separator 925) is flowed into a separator 930 where glycol and the condensed phase amine are separated from the CH₃OH. In some embodiments, the separator 930 is a flash drum. In certain embodiments, the separator 930 is a flash drum operating at a temperature within a range of from 100-150° C., such as 120-140° C., and a pressure of 0.1-1 MPa, such as 0.1-0.5 MPa. The glycol and condensed phase amine are flowed into a distillation column 940 where the glycol and the condensed phase amine are separated. The condensed phase amine optionally is recycled back to the CO₂ absorber 910.

In any of the foregoing embodiments, the process may be a batch process or a continuous process. Some embodiments of the apparatus shown in FIGS. 8A-B and 9A-B may be suitable for a continuous process. In a continuous process, the hydrogenation catalyst may be disposed in a fixed bed in the hydrogenation reactor 820/920, and the H₂ and the condensed phase solution comprising the amine and CO₂ flow through or across the fixed bed. Alternatively, the hydrogenation catalyst may be disposed in a fluidized bed in the hydrogenation reactor 820/920, where the bed is fluidized by the H₂ flow and/or the condensed phase solution, and the H₂ and the condensed phase solution comprising the amine and CO₂ flow through the fluidized bed. In some embodiments, the condensed phase solution is flowed through the hydrogenation reactor at a space velocity of from greater than 0 to 50,000 hr⁻¹, such as from 1-50,000, 100-50,000, 500-40,000, 500-30,000, or 1,000-25,000 hr⁻¹. In some embodiments, a lower space velocity provides an increased conversion of CO₂ to methanol and glycol. In the embodiments of FIGS. 9A-B, the condensed phase solution comprising the CO₂, amine, and epoxide may be flowed through the cycloaddition reactor 915 at a space velocity of from greater than 0 to 50,000 hr⁻¹, such as from 1-50,000, 100-50,000, 500-40,000, 500-30,000, or 1,000-25,000 hr⁻¹. In some embodiments, the space velocities through the cycloaddition reactor and hydrogenation reactor are substantially the same.

In an exemplary embodiment, a one-step process for converting CO₂ to methanol and propylene glycol comprises combining propylene oxide, a Ru-based hydrogenation catalyst, CO₂ and H₂ (in a ratio of 2:7) with a condensed phase comprising PEI and THF. The combined components are heated to 140° C. at an initial pressure of 7 MPa for 24 h.

In another exemplary embodiment, a two-step process for converting CO₂ to methanol and propylene glycol comprises combining propylene oxide and CO₂ with a condensed phase comprising PEI and THF at an initial pressure of 2 MPa, and heating to 140° C. for 16-24 h to form propylene carbonate. Subsequently, the reactor is cooled to 0° C. and pressure is released. A Ru-based hydrogen catalyst and H₂ (CO₂:H₂ molar ratio 1:3) are added to an initial pressure of 6 MPa, and the components are heated to 140° C. for 16 h to form methanol and propylene glycol.

In yet another exemplary embodiment, a two-step process for converting CO₂ to methanol and propylene glycol comprises combining a Ru-based hydrogenation catalyst, propylene oxide, and CO₂ with a condensed phase comprising PEI and THF at an initial pressure of 2 MPa, and heating to 140° C. for 16-24 h to form propylene carbonate. Subsequently, the reactor is cooled to 0° C. and pressure is released. H₂ (CO₂:H₂ molar ratio 1:3) is added to an initial pressure of 6 MPa, and the components are heated to 140° C. for 16 h to form methanol and propylene glycol.

In any of the foregoing exemplary embodiments, the Ru-based hydrogenation catalyst may be a Ru-pincer catalyst. In certain embodiments, the catalyst is

VI. Process for Producing Methane

In some embodiments, an integrated process for capture and conversion of CO₂ to methane includes combining the CO₂, a hydrogenation catalyst, and hydrogen with a condensed phase comprising an amine under conditions effective to form methane and water. The methane is separated from the amine, water, and any residual H₂ and/or CO₂. The overall process is CO₂₊₄H₂→CH₄+2H₂O. In some embodiments, the reaction proceeds via formamide or formate ester intermediates in the condensed phase.

The reaction conditions and catalyst influence whether the reaction produces primarily methanol, as discussed in section IV, or methane. As shown in FIG. 10, CO₂ is combined with the condensed amine phase in the presence of hydrogen to produce a formate intermediate (1), which condenses to produce a formamide intermediate (2). Subsequent catalyst selection, hydrogen stoichiometry, and temperature influence whether the formamide intermediate (2) is hydrogenated to form an N-methylated product, methane, or methanol.

In any of the foregoing or following embodiments, prior to combination with the CO₂, hydrogenation catalyst, and hydrogen, the condensed phase may comprise, consist of, or consist essentially of the amine. In one embodiment, the condensed phase consists of the amine. The condensed phase may consist of the amine when the amine is a liquid at ambient temperature or under the conditions at which the amine and CO₂ are combined. In an independent embodiment, the condensed phase consists essentially of the amine. In another independent embodiment, the condensed phase is a solution comprising the amine and a nonaqueous solvent. In still another embodiment, the condensed phase comprises the amine and from 0-10 wt % water, such as from 0-5 wt % water or 0-2 wt % water. In some examples, the condensed phase is substantially devoid of water prior to the conversion of CO₂ to methanol, e.g., the condensed phase prior to reaction may comprise less than 1 wt % water, prior to reaction.

In any of the foregoing or following embodiments, the amine may be a compound according to Formula I, i.e., a CO₂BOL. In certain embodiments, the condensed phase consists of or consists essentially of the CO₂BOL. By “consists essentially of” or “consists of” is meant that no additional solvent is affirmatively added to the CO₂BOL. By “consists essentially of” is further meant that, prior to CO₂ capture and conversion, the condensed phase comprises at least 95 wt % of the CO₂BOL and comprises less than 1 wt % of any other amine. The condensed phase may be substantially devoid of water prior to the conversion of CO₂ to methanol, e.g., the condensed phase prior to reaction may comprise less than 5 wt % water, such as less than 1 wt % water, prior to reaction. The absence of a cosolvent in some embodiments maximizes the amine concentration and/or the concentration of captured CO₂, and concomitantly reduces the volume of the condensed phase and the energy required to heat the condensed phase to a desired temperature for CO₂ capture and subsequent conversion. The absence of a cosolvent also may simplify the downstream solvent separation and recovery unit. In some embodiments, the CO₂BOL is N-(2-ethoxyethyl)-3-morpholinopropan-1-amine (EEMPA).

In any of the foregoing or following embodiments, a concentration of CO₂ in the condensed phase comprising the amine may be from 1 wt % to 25 wt % CO₂, such as 2 wt % to 20 wt %, 5 wt % to 20 wt %, 5 wt % to 15 wt %, or 8 wt % to 12 wt %. Alternatively a concentration of CO₂ in the condensed phase comprising the amine may be from 1 mol % to 50 mol %, such as 5 mol % to 50 mol %, 10 mol % to 50 mol %, 20 mol % to 50 mol %, 25 mol % to 50 mol %, 25 mol % to 40 mol %, or 30 mol % to 40 mol %.

In any of the foregoing or following embodiments, the hydrogenation catalyst may comprise Ru, Ni, Fe, Co, Rh, or a combination thereof. In some embodiments, the hydrogenation catalyst comprises Ru, Ni, Fe, Co, Rh, or a combination thereof supported on materials such as Al₂O₃, carbon, In₂O₃, CeO₂, TiO₂, SiO₂, ZrO₂, magnesium aluminum spinels (e.g., MgO—Al₂O₃), chromite (e.g., FeCr₂O₄), and combinations thereof. Exemplary catalysts include, but are not limited to, Ru/Al₂O₃ and Ru/C. In some embodiments, the catalyst comprises 3 wt % to 7 wt % Ru on Al₂O₃ or C (e.g., activated charcoal). In certain examples, the catalyst is 5 wt % Ru/Al₂O₃ or 5 wt % Ru/C.

In any of the foregoing or following embodiments, a molar ratio of H₂ to CO₂ may be from 4:1 to 10:1. In some embodiments, the molar ratio of H₂ to CO₂ is from 4:1 to 6:1, or 4:1 to 5:1.

FIG. 11 shows one exemplary embodiment of an integrated process for converting CO₂ to methane. CO₂, a hydrogenation catalyst, hydrogen, and a condensed phase comprising an amine are combined (1110). The combined components are subjected to an effective temperature T_(M) and initial pressure P_(M) for a sufficient time t_(M) to form a solution comprising CH₄, H₂O, and the condensed phase amine (1120). In this step, carbon dioxide is captured by the amine in the condensed phase and is then hydrogenated to form methane and water. By “captured” is meant that the CO₂ is adsorbed, absorbed, covalently bound (e.g., in the form of an alkylcarbonic acid or alkylcarbamic acid), or ionically bound to the amine in the condensed phase. In some embodiments, CO₂ capture is performed at a temperature 60° C., such as a temperature within a range of 25−60° C. In certain embodiments where T_(M)>60° C., a period of time is allowed for CO₂ capture before increasing the temperature to T_(M). The solution comprising the condensed phase amine, CH₄, and H₂O may be cooled to facilitate subsequent separation. In some embodiments, the solution is cooled to a temperature of 25° C. to 50° C., such as 30° C. to 40° C. prior to separation. A separation is performed to provide CH₄ (1130) and a solution comprising the condensed phase amine and H₂O (1140). Separation may be performed by any suitable means. In some embodiments, the CH₄ is recovered in a flash drum by vapor-liquid separation. The condensed phase amine and H₂O may further be separated to provide H₂O (1150) and the condensed phase amine (1160), which optionally is recycled and reused.

In any of the foregoing or following embodiments, the temperature T_(M) may be 100° C. to 200° C., such as 100° C. to 190° C., 110° C. to 190° C., 120° C. to 190° C., 120° C. to 180° C., or 120° C. to 170° C. In any of the foregoing embodiments, the initial pressure P_(M) may be from 0.1 MPa to 10 MPa. In some embodiments, the initial pressure is from 0.1 MPa to 6 MPa, 0.5 MPa to 6 MPa, 1 MPa to 6 MPa, 1 MPa to 4 MPa, or 1 MPa to 3 MPa. In any of the foregoing or following embodiments, the time t_(M) may be within a range of from 3 seconds to 36 hours, such as from 15 seconds to 36 hours, 30 seconds to 36 hours, 1 minute to 36 hours, 5 minutes to 36 hours, 10 minutes to 36 hours, 30 minutes to 36 hours, 1-36 hours, 3-36 hours, 3-24 hours, 3-18 hours, 3 seconds to 3 hours, 3 seconds to 1 hour, 5 seconds to 1 hour, 10 seconds to 30 minutes, or 15 seconds to 15 minutes. In a continuous process, the time t_(M) may be on the order of a few seconds to several minutes, whereas the time t_(M) may be on the order of several hours in a batch process. As set forth above, in certain embodiments where T_(M)>60° C., a period of time is allowed for CO₂ capture before increasing the temperature to T_(M). The period of time for initial CO₂ capture may range from a few seconds to several hours, with longer times being used in batch processes.

In an independent embodiment, the CO₂ is present in the condensed phase with the amine prior to combination with hydrogen and the hydrogenation catalyst. In certain examples, the conversion of CO₂ to methane is a two-step process. As shown in FIG. 12, CO₂ is combined with a condensed phase amine (1202) under effective conditions to form a condensed phase comprising the amine, the CO₂, and optionally a solvent (1204). The effective conditions include a temperature T′, an initial pressure P′, and time t′ sufficient for the condensed phase amine to capture CO₂, thereby providing a condensed phase comprising the amine and captured CO₂. In other embodiments, the process may commence with a condensed phase comprising the amine and captured CO₂ (1204). The condensed phase comprising the amine and CO₂ is combined with H₂ and the hydrogenation catalyst (1210). The combined components are subjected to an effective temperature T_(M) and initial pressure P_(M) for a sufficient time t_(M) to form a solution comprising CH₄, H₂O, and the condensed phase amine (1220), where T_(M), P_(M), and t_(M) are as described above. As described above, a separation is performed to provide CH₄ (1230) and a solution comprising the condensed phase amine and H₂O (1240). The condensed phase amine and H₂O (1240) may further be separated to provide H₂O (1250) and the condensed phase amine (1260), which optionally is recycled and reused. The separation may be performed by any suitable method as set forth above. In any of the foregoing or following embodiments, the temperature T′, initial pressure P′, and the time t′ are as previously described.

FIG. 13A is a schematic diagram of one embodiment of an apparatus 300 for performing the method of FIG. 12. A condensed phase comprising an amine and a gas flow comprising CO₂ are flowed into a CO₂ absorber 1310. The condensed phase comprising the amine and the CO₂ are subjected to a temperature T′, an initial pressure P′, and time t′ sufficient for the amine to capture the CO₂ to form a condensed phase solution comprising the amine and CO₂. A CO₂-depleted gas is exhausted from the CO₂ absorber 1310. The condensed phase solution flows into a hydrogenation reactor 1320 where it is combined with H₂ and a hydrogenation catalyst disposed within the hydrogenation reactor 1320. In any of the foregoing embodiments, the hydrogenation catalyst may be disposed in a fixed bed or a fluidized bed within the hydrogenation reactor 1320. If the bed is a fluidized bed, the H₂ flow and/or the condensed phase solution may be used for the fluidization. The combined components are subjected to an effective temperature T_(M) and initial pressure P_(M) for a sufficient time t_(M) to form a condensed phase solution comprising CH₄, H₂O, and the amine. In any of the foregoing embodiments, the condensed phase solution, the hydrogen, or both may be pre-heated to a temperature ranging from greater than ambient to T_(M) before entering the hydrogenation reactor. In some embodiments, the H₂ flowing into the hydrogenation reactor 1320 is in stoichiometric excess relative to the CO₂ in the condensed phase, and excess H₂ exiting the hydrogenation reactor optionally is recycled back into the hydrogenation reactor. In certain embodiments, e.g., as shown in FIG. 13B, the excess H₂ and the condensed phase solution comprising the CH₄, H₂O, and amine exiting the hydrogenation reactor 1320 are flowed into a separator 1325 where the H₂ is separated from the solution and subsequently recycled to the hydrogenation reactor 1320. In some embodiments, the separator 1325 is a flash drum. Because the methanation reaction is highly exothermic, the reactor temperature is maintained by cooling water, which turns into steam for power generation. The generated power may be used, for example, to preheat the feed flowing into the hydrogenation reactor 1320. Thus, in some embodiments, the hydrogenation reactor 1320 has a tube-in-shell design where cooling water flows into the shell. Reaction heat is transferred into the cooling water, converting the water into steam, which can be used for power generation. In an alternative embodiment, the reactor 1320 has a tube-in-shell design where the condensed phase solution and H₂ flow through the shell and cooling water flows through the tube. The condensed phase solution comprising the CH₄, H₂O, and the amine exiting the hydrogenation reactor 1320 (or separator 1325) is flowed into a vapor-liquid separator 1330 where the CH₄ is separated from the condensed phase amine and H₂O. The condensed phase solution may be cooled before entering the vapor-liquid separator 1330, e.g., to a temperature within a range of from 20° C. to 50° C. In some embodiments, the vapor-liquid separator 1330 is a flash drum. In certain embodiments, the separator 1330 is a flash drum operating at a temperature within a range of from 20° C. to 50° C., such as 30° C. to 50° C., or 30° C. to 40° C., and a pressure of 0.5 MPa to 2 MPa, such as 1 MPa to 2 MPa. The condensed phase amine and H₂O are flowed into a separator 1340 where the condensed phase amine and H₂O are separated. The separator 1340 may comprise a dehydration membrane, and may separate the condensed phase amine and H₂O by a pressure-driven membrane process. The separator 1340 may be operated at a temperature within a range of 20° C. to 50° C., such as 30° C. to 40° C., and an inlet pressure within a range of 0.5 MPa to 2 MPa, such as 1 MPa to 2 MPa. Because the solution entering the separator 1340 is at elevated pressure, additional energy is not needed for the separation. The condensed phase amine optionally is recycled back to the CO₂ absorber 1310.

In any of the foregoing embodiments, the process may be a batch process or a continuous process. Some embodiments of the apparatus shown in FIGS. 13A and 13B may be suitable for a continuous process. In a continuous process, the hydrogenation catalyst may be disposed in a fixed bed in the hydrogenation reactor 1320, and both H₂ and the condensed phase solution comprising the amine and CO₂ flow through or across the fixed bed. Alternatively, the hydrogenation catalyst may be disposed in a fluidized bed in the hydrogenation reactor 1320, where the bed is fluidized by the H₂ flow and/or the condensed phase solution, and the H₂ and the condensed phase solution comprising the amine and CO₂ flow through the fluidized bed. In some embodiments, the condensed phase solution is flowed through the hydrogenation reactor at a space velocity of from greater than 0 to 50,000 hr⁻¹, such as from 1-50,000, 100-50,000, 500-40,000, 500-30,000, or 1,000-25,000 hr⁻¹, or at a flow rate within a range of 100-50,000 tonne hr⁻¹, such as 500-25,000, 1,000-20,000, or 5,000-10,000 tonne hr⁻¹. Alternatively, the flow rate may be expressed as a weight hourly space velocity. In some embodiments, the WHSV is 0.001 g_(CO2)/g_(catalyst) h⁻¹ to 10 g_(CO2)/g_(catalyst) h⁻¹, such as 0.01 g_(CO2)/g_(catalyst) h⁻¹ to 5 g_(CO2)/g_(catalyst) h⁻¹, 0.05 g_(CO2)/g_(catalyst) h⁻¹ to 1 g_(CO2)/g_(catalyst) h⁻¹, or 0.05 g_(CO2)/g_(catalyst) h⁻¹ to 0.5 g_(CO2)/g_(catalyst) h⁻¹. In some embodiments, a lower space velocity or flow rate provides an increased conversion of CO₂ to methane.

In an exemplary embodiment, CO₂ is combined with a condensed phase comprising N-(2-ethoxyethyl)-3-morpholinopropan-1-amine (EEMPA), a hydrogenation catalyst (e.g., Ru/Al₂O₃ or Ru/C) and excess hydrogen relative to an amount of the CO₂ captured by the amine in the condensed phase. The combined components are heated to 120° C. to 170° C. for 3-24 hours, thereby converting CO₂ to methane and water.

In any of the foregoing or following embodiments, CO₂ captured by the amine may be the limiting reactant, and at least 25 mol %, at least 50 mol %, at least 75 mol %, at least 90 mol %, or even at least 95 mol % of the captured CO₂ is consumed. For example, from 25-100, 50-100, 75-100, 90-100, or 95-100 mol % of the captured CO₂ may be consumed. Alternatively, at least 10 mol %, at least 25 mol %, at least 50 mol %, or at least 75 mol % of CO₂ combined with the hydrogen, hydrogen catalyst, and condensed phase comprising the amine is consumed. For example, from 10-100, 25-100, 50-100, or 75-100 mol % of the CO₂ may be consumed.

In any of the foregoing or following embodiments, the methane selectivity may be at least 80%, at least 85%, at least 90%, or at least 95%, such as 80-98%, 85-98%, or 85-95%. In other words, at least 80% of the consumed CO₂ is converted to methane. Byproducts may include methanol, ethane, ethanol, and higher hydrocarbons. In any of the foregoing or following embodiments, the captured CO₂ conversion to methane may be at least 15%, at least 20%, at least 25%, at least 30%, at least 35%, at least 40%, at least 45%, or at least 50%. In one example, the amine was EEMPA, T_(m) was 170° C., P_(m) was 1.5 MPa, t_(m) was 3 hours, and the process converted >50% of the captured CO₂ with 87% methane selectivity and no detectable amount of methanol. Increasing the reaction time increases the conversion percentage.

Advantageously, in some embodiments, the disclosed method may increase the thermal efficiency of the methanation process (e.g., by 5% or more) compared to the conventional flue gas to synthetic natural gas process in which CO₂ in the flue gas is captured in an absorber, released as in a stripper, and then compressed and mixed with H₂ for methanation at around 350° C. and 3 MPa as described previously. The energy consumption required for CO₂ pressurization in the conventional process is reduced by 60-70%. Consequently, the process cost is also reduced.

VII. Examples

General:

All manipulations were performed using a N₂-atmosphere glovebox or standard Schlenk techniques. The catalysts 1, 2 and 3 were obtained from Strem chemicals. Standard solvents were purified by passing through a neutral alumina column (Innovative Technology, Inc., PureSolv™ solvent purification system). All deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc. Anhydrous propylene oxide was purchased from Sigma-Aldrich. All other materials were purchased from commercial suppliers and used without further purification unless otherwise mentioned. Monoethanolamine (MEA) and diethylethanolamine (DEEA) were distilled over CaH₂ and stored in the glovebox. 4-Dimethylaminopyridine (DMAP) and polyethyleneimine M_(n) 600 (PEI₆₀₀) were dried under vacuum overnight and stored in the glovebox. ¹H and ¹³C NMR experiments were recorded on a 500 MHz Varian NMR spectrometer. The NMR chemical shifts were assigned relative to the internal standard, 1,3,5-trimethoxybenzene (TMB). The headspace gas was analyzed using an Agilent Technologies 6850 GC System equipped with a Supelco 10 ft×⅛ inch carbosieve column equipped with a thermal conductivity detector (TCD). MAS-NMR experiments were performed on an Agilent-Varian VNMRS NMR spectrometer equipped with an 11.7 T magnet, operating at 125.7747 MHz for the ¹³C channel and 500.1822 MHz for ¹H channel, and using a 5 mm homebuilt MAS double-resonance HX probe with a custom Pd-coated coil for increased sample magnetic homogeneity. The rotor was heated to various set temperatures and both ¹³C and ¹H spectra were collected before further temperature elevation. All rotors were Varian/Agilent-style cavern rotors (Revolution NMR LLC), modified for high pressure samples as described previously (Walter et al., J Phys Chem C 2018, 122(15):8209-8215). A premixed solution containing propylene oxide (240 mg), PEI₆₀₀ (60 mg), catalyst 3 (2.4 mg) and THF-d₈ (1 mL) was transferred to a MAS NMR rotor in a N₂-atmosphere glovebox. The rotor was charged with a 15 bar CO₂ and 60 bar H₂ at room temperature (total initial pressure=60 bar) and heated to 140° C.

Example 1 Conversion of CO₂ to Methanol

CO₂ can be captured from air or concentrated sources by amine, amine/H₂O, or an amine/alcohol mixture to form carbamate, bicarbonate, or carbonate, respectively (FIG. 10). The CO₂ activated in this fashion can be utilized to produce CO₂-derived products, such as methanol.

As a catalyst, 300 mg of commercial Cu/ZnO/Al₂O₃catalyst from Synetix was activated under H₂ atmosphere (48 bar, 4.8 MPa) at 125° C. in a stainless-steel reactor (8 mL). After 4 h, the remaining H₂ pressure from the reactor was released and backfilled with H₂ at 125° C. and cooled to room temperature. The commercial black Cu/ZnO/Al₂O₃catalyst's color changed to purple-black powder after the activation. The catalyst contained 55.7 wt. % Cu, 26.8 wt. % Zn, and 5.0 wt. % Al according to inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis.

The pre-activated catalyst was transferred to a 300 mL Parr reactor and a given amount of amine and alcohol were added to the reactor vessel and the reactor was sealed in a nitrogen glovebox. The reactor was first pressurized with 20 bar (2 MPa) CO₂ and then additional 40 bar (4 MPa) of H₂ was introduced (total pressure=60 bar, 6 MPa). Then, the reactor was heated to 170° C. After 16 h, the reactor was cooled to room temperature and then to −78° C., the excess pressure was released slowly. 100 mg of 1,3,5-trimethoxybenzene was added as an internal standard to the reaction mixture (if necessary, DMSO or water was added to get a homogeneous solution) and a small aliquot of the sample was analyzed by ¹H and ¹³C NMR in CDCl₃ or D₂O. The results are shown in Table 1.

TABLE 1 Entry Promoter HCOO⁻ (%) HCOOEt (%) CH₃OH (%)  1 NEt₃ Trace — —  2 EtOH — — —  3 NEt₃:EtOH 1:1 Trace Trace 2%  4 NEt₃:EtOH 10:1 — — —  5 NEt₃:EtOH 1:10 3% Trace 100%   6^(a) NEt₃:EtOH 1:10 3% 1% 76%  7^(b) NEt₃:EtOH 1:10 — 1% 28%  8^(c) NEt₃:EtOH 1:10 4% 4% 19%  9 NEt₃:EtOH 1:5 Trace Trace  7% 10 TMEA:EtOH 1:10 — 1% 18% 11 DEEA:EtOH 0.5%   Trace 21% EtOH = ethanol; NEt₃ = triethylamine; TMEA = tetramethylethylenediamine, DEEA = diethylethanolamine. Standard reaction conditions: 300 mg Cu/ZnO/Al₂O₃ catalyst, CO₂/H₂ = 60 bar (6 MPa, 1:2), EtOH (200 mmol), amine (20 mmol), T = 170° C., t = 16 h. HCOO⁻, HCOOEt, and CH₃OH yields are based on ¹H NMR. Yields are calculated with respect to the amine. ^(a)40 h, ^(b)150° C., ^(c)120° C.

An initial screening showed that 1° and 2° amines (e.g., ethylenediamine and dibutylamine) were hydrogenated to formamides, but were unable to continue to CH₃OH. Tertiary amines, e.g., NEt₃, were found to be nonreactive, as were alcohols. However, the combination of 3° and alcohol produced CH₃OH (entries 1-3, Table 1). These results suggested that the neutral CO₂ might not be the active species. The tertiary alkanoiamines, N,N,N′,N′-tetrakis(2-hydroxyethyl)ethylenediamine (THEED) and triethanolamine (TEA) decomposed at a high temperature. Diethylethanolamine (DEEA), however, produced 6.5 mmol of CH₃OH (4% conversion based on amine). Changing the ratio of amine to alcohol influenced the conversion to CH₃OH; when the ratio of amine to alcohol was 10:1, the conversion to CH₃OH with respect to amine was poor, whereas a ratio of 1:10 enabled a 100% conversion to CH₃OH with respectto amine (entries 4 and 5, Table 1). A CH₃OH synthesis activity as high as 4166 mmol kg⁻¹ cat⁻¹ h⁻¹ was obtained at 170° C. with a CH₃OH yield of ˜10% with respect to CO₂. An excess of NEt₃ could passivate the catalyst surface, limiting the reaction. It is likely that polarity has a strong influence on the reactivity as we have shown the formation of alkylcarbonates to be highly sensitive with respect to polarity. The relatively lower polarity of NEt₃ would disfavor the formation of the highly polar alkylcarbonate, whereas the polarity of excess alcohol would be enough to favor alkylcarbonate formation. Further EtOH could solvate polar transition states and charged intermediates common to CO₂ hydrogenation whereas NEt₃ would not. Excess alcohol would also promote the thermal esterification to produce formate ester. Gas chromatographic analysis of the gas mixtures only showed trace amounts of CH₄ and CO in addition to excess H₂ and CO₂.

The necessity for the combination of non-nucleophilic bases and alcohols suggests that CH₃OH production proceeds via formate and formate ester routes. The presence of amine and alcohol promotes the formation of alkylcarbonate, ammonium formate and alkyl formate ester intermediates. Under optimal conditions, the amount of CH₃OH produced never surpassed the amount of amine used regardless of time, which suggested an established equilibrium (entry 6, Table 1). Even at lower temperatures 150° C. and 120° C., moderate CH₃OH yields were obtained (entries 7 and 8), suggesting that the reaction does not necessarily need to proceed at 170° C. Tetramethylethylenediamine (TMEA) and DEEA formed 18% and 21% CH₃OH, respectively in the presence of excess ethanol (entries 10 and 11). It was clear that excess alcohol does not improve the amount of CH₃OH formed in the case of DEEA as it decomposes under these conditions.

The effect of basicity on the alkylcarbonate formation capacity of amines and alcohols on the CH₃OH formation was then studied. The alkyl carbonate formation in Table 2 was studied under high pressure since some alkylcarbonates are not stable under atmospheric conditions. Under an inert atmosphere, 0.0012 mol of amine was dissolved in 3.0 mL of methanol. The solution was syringed into the stainless-steel IR cell or into a PEEK high pressure NMR cell and charged with 1 MPa CO₂ at room temperature. The NMR cells were agitated by vortex until equilibrium has been reached.

From Table 2, it is clear that the strong bases provide high methyl carbonate yields, but low CH₃OH yields (FIG. 11). Meanwhile, less basic amines gave moderate methyl carbonate yields but high CH₃OH yields. NEt₃ showed the highest CH₃OH yield, suggesting that having a conjugate acid of the amine that is weaker than the corresponding alkylcarbamic or alkylcarbonic acids in H₂O is optimal under these conditions. Since aromatic ring hydrogenation was observed in the presence of the Cu/ZnO/Al₂O₃catalyst, DMAP and pyridine were not selected for further study.

TABLE 2 Methyl IR ¹³C NMR Conjugate carbonate (CO) (ppm) Entry Amine Acid pK_(a) (%) cm⁻¹ I^(a) II^(b) 1 DBU 12 98 1642 52.2 159.8 2 DIPEA 11.4 92 1647 51.1 158.8 3 NEt₃ 10.6 75 1654 52.3 160.1 4 DMAP 9.9 64 1651 52.4 160.5 5 DABCO 8.8 64 1650 49.1 160.0 6 Pyridine 5.5 No reaction Standard reaction conditions: alkylcarbonate formation from CO₂ capture-alkylcarbonate yield calculated with respect to amine by ¹³C NMR of 2M amine in CH₃OH under 10 bar (1 MPa) CO₂ at room temperature. ^(a)NMR resonance of methyl carbon in CH₃OCOO⁻ species. ^(b)NMR resonance of carbonate carbon in CH₃OCOO⁻ species. DBU = 1.8-diazabicyclo[5.4.0]undec-7-ene, DIPEA = N,N-diisopropylethylamine, DMAP = 4-dimethylaminopyridine, DABCO = 1,4-diazabicyclo[2.2.2]octane.

Similarly, the choice of alcohol was investigated under pressure (Table 3). Under an inert atmosphere, 0.0012 mol of NEt₃ was dissolved in 3.0 mL of alcohol. The solution was syringed into a PEEK high pressure NMR cell and charged with 1 MPa CO₂ at room temperature. Under an inert atmosphere, 0.0012 mol of NEt₃ and 0.0012 mol of phenol were dissolved in 3.0 mL of THF. The solution was syringed into a PEEK high pressure NMR cell and charged with 1 MPa CO₂ at room temperature.

The presence of CH₃OH or EtOH did not show significant difference in the alkylcarbonate yield, which was attributed to similar pK_(a) values of the alcohols. However, the alkyl chain length and sterics significantly reduced the alkylcarbonate conversion and CH₃OH yields (FIG. 11). This result was consistent with previous observations that secondary alcohols do not form as much alkylcarbonate as linear alcohols due to steric crowding. Similarly phenol likely is not electrophilic enough to attack CO₂ and does not carboxylate under the evaluated conditions. The results obtained in Tables 2 and 3 and FIG. 11 suggested that the tertiary amines and short linear alcohols were the best combination for high CH₃OH yields, consistent with the conditions that favor alkylcarbonate formation.

TABLE 3 Alkyl Entry Alcohol (%) pKa carbonate (%) 1 CH₃OH 16 75 2 EtOH 15.9 75 3 i-Propanol 16.5 25 4 Ethyleneglycol^(a) 14.2 35 5 t-Butanol 17 No reaction 6 Phenol^(b) 10 No reaction Standard reaction conditions: alkylcarbonate formation from CO₂ capture-alkylcarbonate yield calculated with respect to amine by ¹³C NMR of 2M NEt₃ in alcohol under 10 bar (1 MPa) CO₂ at room temperature. ^(a)Only one OH group is carbonated. ^(b)The reaction was performed in THF.

While a direct correlation between the alkylcarbonate formed and CH₃OH formation was observed (Tables 2 and 3), it is expected that there is a small amount of alkylcarbonate present at 170° C. under 2.5 MPa CO₂. A plausible reaction mechanism via the coordination of anionic ethyl carbonate to the catalyst surface, by which the reaction proceeds via formate and ethyl formate intermediates is proposed (FIG. 12).

Catalyst recycling was studied under 1:2 CO₂:H₂pressure. In the second run, some drop in catalytic activity was observed (FIG. 13). However, the activity remained the same in the third run. The decrease in the catalytic activity was probably due to the catalyst sintering, which was confirmed by powder X-ray diffraction (XRD) performed after the third run. X-ray diffraction (XRD) patterns were obtained using a Rigaku MiniFlex II powder x-ray diffractometer with a Cu-kα x-ray source operated at 30 kV and 15 mA with an amorphous glass sample holder. Prior to measurement, the fresh sample was ground and reduced at 125° C. for 4 h under 48 bar H₂ atmosphere. The spent sample was collected and used as recovered. XRD patterns were collected from 10 to 80° 2θ at a step size of 0.02° 2θ/step. Peaks were analyzed and fit using the MDI Jade 9 software. Cu⁰ crystallite size was calculated using the Scherer equation based on the 43.5° 2θ peak. Cu dispersion was calculated from the Cu⁰ particle size assuming hemispherical geometry using the formula D=100/d where D represents the fractional dispersion and d is the Cu⁰ particle size (in nm). The Cu particle size increased from 21 nm (freshly reduced) to 40 nm (spent). Similar sintering of the catalyst in a batch reactor was observed previously in the presence of H₂O, the by-product. XRD analysis also showed the presence of ZnCO₃, which could have formed from an excess CO₂ concentration under pressurized conditions. Thus, changes in activity could be explained by both sintering of Cu particles and the phase change of the ZnO support. Inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis of the solution after hydrogenation showed minimal catalyst leaching (Cu=2.2 ppm, Zn=5.1 ppm, Al 1.3 ppm).

MAS-NMR experiments were performed on an Agilent-Varian VNMRS NMR spectrometer equipped with an 11.7 T magnet, operating at 125.7747 MHz for the ¹³C channel and 500.1822 MHz for ¹H channel, and using a 5 mm homebuilt MAS double-resonance HX probe with a custom Pd-coated coil for increased sample magnetic homogeneity. The rotor was heated to various set temperatures and both ¹³C and ¹H spectra were collected before further temperature elevation. All rotors were Varian/Agilent-style cavern rotors (Revolution NMR LLC), modified for high pressure samples. 1:10 NEt₃:EtOH mixture (0.09 mmol:0.9 mmol) and 4.7 mg of pre-activated Cu/ZnO/Al₂O₃were transferred to a MAS NMR rotor in a N₂ glovebox. The rotor was charged with 2 MPa CO₂ at room temperature. Then the NMR of the initial reaction mixture was taken at 25° C., which showed the formation of triethylammonium ethylcarbonate (EtOCO⁻Et₃NH⁺) upon the reaction of free CO₂ with NEt₃ and ethanol. Subsequently the temperature was increased to 55, 78 and 120° C. The percentage of EtOCO⁻Et₃NH⁺ with respect to total amine content vs. temperature was plotted (FIG. 14).

Next, 4 MPa of hydrogen was introduced and the MAS rotor was heated at 170° C. for 8 h. The high-temperature operando ¹³C magic angle spinning (MAS) NMR study of the catalyst in the presence of 1:10 NEt₃ and EtOH mixture at 2 MPa CO₂ at 120° C. showed ethylcarbonate (158.9 ppm) at a concentration 2.2 times that of the active sites of the catalyst. As shown in Table 1, 19% conversion to CH₃OH was observed at 120° C., indicating that ethylcarbonate exists in a significant enough concentration to participate in the reaction at least up to this temperature.

Operando ¹³C MAS NMR at 170° C. in the presence of H₂ did not show a detectable amount of ethylcarbonate, though it clearly showed the formation of triethylammonium formate (168.5 ppm). The CH₃OH signal at 48.7 ppm started to increase quickly in less than 5 minutes of heating at 170° C. (FIG. 15). Over time CH₃OH decomposed into CO (182.2 ppm), confirming that the hydrogenation does not proceed via CO.

The CO₂ hydrogenation via the ammonium formate and alkyl ester intermediates in the presence of a Pd/ZnO catalyst under the same conditions was also studied. 1:10 NEt₃:EtOH mixture (0.09 mmol: 0.9 mmol) and 4 mg of pre-activated Pd/ZnO were transferred to a MAS NMR rotor in a N₂ glovebox. The rotor was charged with a 2 MPa CO₂ and 4 MPa of hydrogen at room temperature and the MAS rotor was heated at 170° C. for 15 h. There was no detectable amount of CH₃OH observed by ¹³C NMR at 170° C. (FIG. 16). However, slow accumulation of alkyl formate was observed over time.

Low-temperature condensed phase heterogeneous hydrogenation of CO₂ to CH₃OH using NEt₃ and EtOH was demonstrated. The formation of side products such as CO and CH₄ was significantly reduced. Screening of various amines and alcohols revealed that alkyl carbonate, ammonium formate and alkyl ester were the key intermediates involved in the reaction which was confirmed by operando ¹³C MAS NMR. The degree of alkyl carbonate and methanol formation was found to be limited by the solvent polarity as well as the basicity of the amine. Performing the present reaction in a flow system could significantly improve the overall productivity of the system, as poisoning of catalysts arising from the accumulation of products would be minimal.

Example 2 Cycloaddition of CO₂ to Epoxides

The formation of cyclic carbonates by addition of CO₂ to epoxides in a condensed phase comprising an amine was evaluated. A 100 mL Parr reactor was charged with a given amount of epoxide (propylene oxide, PO) and amine and sealed in a N₂-atmosphere glovebox. The reactor was then pressurized with 30 bar (3 MPa) CO₂ and heated to a set temperature for a given time. After the set time, the reactor was cooled to room temperature and then to 0° C., the excess pressure was released slowly. TMB (1,3,5-trimethoxybenzene, 100 mg) was added as an internal standard to the reaction mixture and a small aliquot of the sample was analyzed by ¹H and ¹³C NMR experiments in CDCl₃. (Amidines were not used for this study because they tend to degrade at elevated temperatures under reductive conditions.) The reaction is shown below:

First, MEA, the most commonly used post-combustion CO₂ capture solvent, was studied for the formation of cyclic carbonate (entry 1, Table 4). MEA produced a very small amount of cyclic carbonate at 110° C. When a pre-combustion capture solvent, diethyl ethanol amine (DEEA), was used (entry 2, Table 4), only traces of cyclic carbonate was observed by ¹H NMR experiment.

TABLE 4 Temperature ² PC Entry ¹ Amine (° C.) Conversion (%) 1 MEA (10 mol %) 110 0.2 2 DEEA (5 mol %) 110 traces 3 DMAP (10 mol %) 110 58 4 PEI₆₀₀ (300 mg) 110 54 5 PEI₆₀₀ (100 mg) 110 11 6 PEI₆₀₀ (100 mg) 25 0 7 PEI₆₀₀ (100 mg) 140 97 ¹ Amines were in THF. ² PC conversion % is relative to the epoxide amount.

4-dimethylaminopyridine (DMAP) was identified as one of the reactive bases that ring open epoxides via epoxide activation pathway (Scheme 2). Under our reaction conditions, a cyclic carbonate conversion of 58% was obtained with a good selectivity for PC (entry 3, Table 4). A high boiling polyamine (PEI₆₀₀) was also screened under the same reaction condition, and a cyclic carbonate conversion of 54% was achieved. Unlike DMAP, which was reported to first activate the epoxide, PEI₆₀₀ is expected to activate the CO₂, first via CO₂ activation pathway (Scheme 2) and subsequent nucleophilic attack of the carbanion on the epoxide, opens the ring and cyclizes to carbonate.

Upon the reaction of CO₂ with amines similar to PEI₆₀₀, a carbamic acid species, [—HN⁺CO₂ ⁻] is first formed, which then exists in equilibrium with carbamate, [—NH⁺][—NCO₂ ⁻]. Therefore, in addition to the CO₂ activation pathway described in Scheme 2, mechanism A, involving a carbamic acid intermediate, a competing reaction mechanism involving [—NH⁺][—NCO₂ ⁻] ion pair is also expected to occur. A DFT calculation of the reaction mechanism for the formation of cyclic carbonate from CO₂ and PO in the presence of DBU hexanol mixture suggested that after the initial activation of CO₂, the [DBUH⁺][C₆H₁₃OCO₂ ⁻] ion pair ring opens the epoxide (and not the [C₆H₁₃OCO₂] anion) and forms PC. Similarly, in the case of PEI₆₀₀, the [—NH⁺][—NCO₂ ⁻] carbamate ion pair ring opens the epoxide and liberates PC and regenerates [—NH⁺][—NCO₂ ⁻] carbamate under CO₂ atmosphere.

PEI₆₀₀ was chosen for further optimization. Lowering the PEI₆₀₀ concentration significantly decreased the cyclic carbonate conversion to 11% (entry 5, Table 4). At room temperature, there was no detectable amount of PC observed, and the PO remained unreacted (entry 6, Table 4). Increasing the temperature to 140° C. resulted in increased cyclic carbonate yield of 97% with a selectivity of >99% (entry 7, Table 4).

Example 3 CO₂ Conversion to Methanol and Propylene Glycol

One- and two-step processes for converting CO₂ to methanol and propylene glycol (PG; also known as 1,2-propanediol, 1,2-PD) were evaluated. The reaction and catalyst are shown below. In the one-step process, propylene oxide (PO), a hydrogenation catalyst, and hydrogen were simultaneously added, and the propylene carbonate (PC) formed in step (a) immediately reacted with the catalyst and H₂ in step (b) to form methanol and 1,2-PD. Two-step processes included (i) reaction with PO (step (a)) followed by addition of the hydrogenation catalyst and hydrogen and subsequent reaction to form methanol and 1,2-PD (step (b)), and (ii) reaction with PO in the presence of the hydrogenation catalyst to form PC (step (a)), followed by addition of hydrogen and reaction to form methanol and 1,2-PD (step (b)).

One-pot one-step reaction: A Parr reactor (100 mL) was charged with PO, PEI₆₀₀, metal catalyst and THF, and sealed in a N₂-atmosphere glovebox. The reactor was then pressurized with 20 bar CO₂ and 50 bar H₂ and heated to 140° C. for 24 h (total initial pressure=70 bar). After 24 h, the reactor was cooled to room temperature and then to −78° C., the excess pressure was released slowly. TMB (100 mg) was added as an internal standard to the reaction mixture and a small aliquot of the sample was analyzed by ¹H and ¹³C NMR experiments in CDCl₃.

One-pot two-step reaction: A Parr reactor (100 mL) was charged with PO, PEI₆₀₀ and THF, and sealed in a N₂-atmosphere glovebox. The reactor was pressurized with a 20 bar CO₂ and heated to 140° C. for 24 h. After 24 h, the reactor was cooled to room temperature and then to 0° C., the excess pressure was released slowly. The reactor was opened in the nitrogen atmosphere glovebox and the metal catalyst was added to the reaction mixture. Then the reactor was resealed and pressurized with a 60 bar H₂ and reheated to 140° C. for 16 h. After 16 h, the reactor was cooled to room temperature and then to −78° C., the excess pressure was released slowly. TMB (100 mg) was added as an internal standard to the reaction mixture and a small aliquot of the sample was analyzed by ¹H and ¹³C NMR in CDCl₃.

One-pot two-step reaction—Sequential addition of CO₂ and H₂: A Parr reactor (100 mL) was charged with PO, PEI₆₀₀, metal catalyst and THF, and sealed in a N₂-atmosphere glovebox. The reactor was pressurized with a 20 bar CO₂ and heated to 140° C. for 16 h. After 16 h, the reactor was cooled to room temperature and then to 0° C., the excess pressure was released slowly. Then the reactor was pressurized with a 60 bar H₂ and reheated to 140° C. for 16 h. After 16 h, the reactor was cooled to room temperature and then to −78° C., the excess pressure was released slowly. TMB (100 mg) was added as an internal standard to the reaction mixture and a small aliquot of the sample was analyzed by ¹H and ¹³C NMR experiments in CDCl₃.

Results are shown in Table 5.

TABLE 5 Metal CO₂/H₂ PC PG CH₃OH Entry PO (mmol) Catalyst (MPa) (%) (%) (%) 1^(a) 20 + PEI₆₀₀ — 2/5 96 — — 2^(b) entry 7, Table 4 1 6 87 13 — 3^(b) entry 7, Table 4 2 6 4.8 95.2 69 4^(b) entry 7, Table 4 3 6 0 >99 84 5 20 3 2/5 9 54 17 6^(c) 20 3 2/5 9 78.5 31 7 20 + PEI₆₀₀ 3 2/5 12 84 32 Reaction conditions: THF = 5 g, t = 16 h, PO = 20 mmol, catalyst = 0.02 mmol, PEI₆₀₀ = 100 mg, T = 140° C. ^(a)24 h. Percent yields are relative to the epoxide. ^(b)The final reaction mixture from entry 7, Table 4 was hydrogenated in entries 2, 3, and 4 for 16 h. ^(c)36 h.

In the absence of a metal catalyst and only in the presence of PEI₆₀₀, there was no formation of methanol or glycol (Table 5, entry 1). Several Ru-based catalysts have been identified in the literature for hydrogenation of carbonyl moieties, from which a selected number of catalysts were screened for hydrogenation of in situ formed PC. Among the catalysts screened (Table 5, entries 2-4 and FIG. 17), the Ru-PNP pincer catalyst with the aliphatic backbone (catalyst 3) provided good yields for PG and methanol. The methanol yield was relatively lower than the PG yield because of the loss of methanol during depressurization of the reactor even at reduced temperature. Mass balance was confirmed by recreating the venting procedure and passing the gas through a cold trap to condense methanol. In addition, subjecting a 1:1 mixture of PG and methanol to the experimental reaction conditions (140° C., 16 h, 6 MPa H₂, THF solvent) and venting also resulted in the same glycol to methanol ratio. Gas chromatographic analysis of the vented gas showed only traces of CO from entry 4, Table 5.

Next, steps (a) and (b), which are the PC and PG (and methanol) formation steps, respectively, were combined. Even in the absence of PEI₆₀₀, methanol and glycol were formed, albeit at a lower reaction yield (entry 5, Table 5). Some of the PO remained unreacted (29%), and intermediates such as PC (9%) and formyl esters and amides (11%) were also observed by ¹H NMR experiments. Longer reaction time resulted in improved PG and methanol yields (entry 6, Table 5), and 6.5% of the PO remained unreacted. Interestingly, the concentration of intermediates, PC (9%) and formyl esters and amides (11.5%), produced remained unchanged.

Addition of PEI₆₀₀ increased PG and methanol yields (entry 5 vs entry 7, Table 5). There was no remaining unreacted PO. However, PC (12%) and formyl esters and amides (11%) were observed by ¹H NMR. The addition of a metal catalyst for the second step required disassembly of the pressurized reactor, which may not be economical for practical application. Thus, the metal catalyst was added in step (a) along with PO, PEI₆₀₀, and CO₂. The H₂ was introduced subsequently in step (b). Results are shown in Table 6. Addition of catalysts 2 or 3 did not change the rate of formation of cyclic carbonate (FIG. 18) while catalyst 3 tolerated prolonged heating time during cyclic carbonate formation and remained active for the hydrogenation. The PG and methanol yields of >99% and 82%, respectively, were obtained based on ¹H NMR (FIG. 19).

TABLE 6 PO Metal PC PG CH₃OH Entry (mmol) Catalyst (%) (%) (%) 1^(a) 20 + PEI₆₀₀ 3 0 >99 82 2^(b) 20 + PEI₆₀₀ 2 traces >99 10 Reaction conditions: THF = 5 g, total time including cycloaddition (16 h) and hydrogenation (16 h) steps = 32 h, catalyst = 0.02 mmol, PEI₆₀₀ = 100 mg, CO₂:H₂ = 2 MPa/5 MPa (total pressure = 7 MPa), and T = 140° C. Percent yields are relative to the epoxide.

Without wishing to be bound by a particular theory of operation, a plausible mechanism for the formation of methanol and PG is shown in FIG. 20. Upon initial activation of pre-catalysts 2 and 3 in the presence of a base, activated catalyst species 2a and 3a are formed. Heterolytic cleavage of H₂ on 2a and 3a yields dihydride complexes 2a′-H₂ and 3a′-H₂, respectively. The hydrogenation of P_(C), Int. B, and formaldehyde is catalyzed by dihydride complexes 2a′-H₂ and 3a′-H₂ by a very well investigated metal-ligand cooperation via dearomatization (2a)/aromatization (2a′-H₂) or N—H site deprotonation (3a)/protonation (3a′-H₂) (Kuriyama et al., Org. Process Res. Dev. 2011, 16(1):166-171; Balaraman et al., Nat. Chem. 2011, 3(8):609-614; Han et al., Angew. Chem. Int. Ed. 2012, 51(52):13041-13045; Nielsen et al., J Phys Chem C 2018, 122(15):8209-8215). The hydrogenation of each intermediate species (carbonate, formate ester, and formaldehyde) was already reported in the literature by different groups in the presence of catalysts 2 and 3 (Kothandaraman et al., J Am. Chem. Soc. 2016, 138(3):778-781; Rezayee et al., J Am. Chem. Soc. 2015, 137(3):1028-1031; Han et al., Angew. Chem. Int. Ed. 2012, 51(52):13041-13045; Balaraman et al., Nat. Chem. 2011, 3(8):609-614). The reactivity for these intermediates was expected to decrease with decreasing electrophilicity in the following order: formaldehyde>formate ester (Int. B)>PC (Pritchard et al., Chem. Soc. Rev. 2015, 44(11):3808-3833). The protic hydrogen from the “CH₂” and “NH” cooperative sites of the ligands in dihydride complexes 2a′-H₂ and 3a′-H₂ interact with the carbonyl moieties of PC (Step i), Int. B (Step iii), and formaldehyde (Step v) and assist the nucleophilic attack by Ru—H and subsequent formation of Int. A, Int. C, and the final product, methanol.

Catalyst 3 was previously studied for CO₂ and CO hydrogenation to methanol via a formamide intermediate.

To understand whether this competing reaction was occurring, operando MAS ¹³C NMR was performed at 140° C. under 6 MPa CO₂:3H₂ pressure (FIG. 21). Reaction conditions: 240 mg propylene oxide+60 mg PEI₆₀₀+2.4 mg catalyst 3 in THF-d₈ (1 mL) at 140° C. PC was the first intermediate that was formed, and the methanol concentration increased with increasing PC concentration. Formaldehyde was not observed, which suggests that it undergoes hydrogenation readily. There were multiple small signals that appeared between 158-168 ppm, which were attributed to formate esters and amides. Similar to what was previously observed for the experiments in Table 6 (entries 5 and 6), the concentration of these broadly assigned formate esters and amides did not change with reaction time. Informed simply by the ¹³C NMR, the direct hydrogenation of CO₂ via the formamide pathway could not be disregarded. However, the methanol yields never exceeded the glycol yields in any experiments. In addition, methanol was observed in experiments that did not have PEI₆₀₀ (entries 5 and 6, Table 5) and where the formamide could not be formed. Therefore, hydrogenation of CO₂ via cyclic carbonate (PC) is likely the major pathway. Unlike the formamide route (reaction (a), Scheme 4), no H₂ is wasted in the formation of water as a byproduct. Ultimately, with H₂ recycling, coproduction could result in a theoretical 100% atom efficiency.

Thermodynamically, the coproduction of methanol and glycol (−32.3 kcal/mol) is more favorable than the individual reactions of −11.8 kcal/mol for direct CO₂ hydrogenation to methanol and −20.5 kcal/mol for hydrolysis of propylene oxide to PG. The energetics for the formation of methanol through CO₂ hydrogenation (a) is compared with the coproduction route (b) in FIG. 22. The presence of alcohols and amines is known to promote the formation of methanol in CO₂ hydrogenation as the reaction proceeds via formamide and ester intermediates (Kothandaraman et al., J Am. Chem. Soc. 2016, 138(3):778-781; Rezayee et al., J Am. Chem. Soc. 2015, 137(3):1028-1031; Everett et al., Chem. Commun. 2017, 53(68):9502-9504). The hydrogenation of ester and formamide are the rate limiting steps, which was identified by the inventors and others (Kothandaraman et al., Catal Sci Technol 2018, 8:5098-5103; Rayder et al, Chem 2020, 6:1-13). The N-formamide is 11.8 kcal/mol more stable than the ester intermediate and, accordingly, the ester is relatively more reactive than the formamide during hydrogenation.

A similar thermodynamic stability study of the intermediates involved in the coproduction process showed that propylene carbonate is the most stable intermediate and thus PC was the main intermediate identified by operando ¹³C NMR (FIG. 21). The formaldehyde formation step is thermodynamically uphill from the starting materials (CO₂ and H₂) in the case of direct CO₂ hydrogenation; consequently, excesses of amines and alcohols are necessary to drive the reaction forward although they further complicate the separation process. On the other hand, in the case of the coproduction approach, a 1:1 mixture of glycol and methanol is produced without the need for excess alcohol or amine, therefore easing the separation process.

In conclusion, two commodity chemicals, methanol and propylene glycol, were produced directly from CO₂, H₂, and epoxide for the first time in one pot. PEI₆₀₀ was used for carbon capture and as an organocatalyst for the formation of a cyclic carbonate intermediate that was then hydrogenated in the same pot in the presence of Ru-PNP catalysts to produce methanol and propylene glycol. Operando ¹³C NMR analysis showed the formation of mainly PC and small amounts of ester and amide intermediates under experimental reaction conditions. Unlike the low-temperature (<150° C.) direct CO₂ hydrogenation process in which an excess of alcohols and amines is used, the coproduction approach formed methanol along with glycol in good yields (>95% for PG and 84% for methanol) at 140° C. in the presence of a catalytic amount of amine and the Ru-PNP catalyst. In addition, the coproduction approach reduces the energy-intensive water separation process involved in the individual reactions, and eliminates the needed for added water and/or removal of any byproduct water since water is both generated and consumed in situ.

Example 4 CO₂ Conversion to Methane

A capture solvent as disclosed herein, N-(2-ethoxyethyl)-3-morpholinopropan-1-amine (EEMPA), was selected for bath reactor studies to form methane from CO₂. EEMPA is a single component, water-lean post-combustion CO₂ capture solvent with high durability and low total capture cost. A post-combustion capture solvent is preferred as it can separate CO₂ from dilute streams. Commercial Ru catalysts, Ru/C and Ru/Al₂O₃were screened for hydrogenation of CO₂ captured EEMPA solvent, and compared to Pd/ZnO/Al₂O₃, Pd/ZnO, and Cu/ZnO/Al₂O₃. The temperature ranged from 120° C. to 170° C. The initial hydrogen pressure in each trial was 1.5 MPa. Instead of using gaseous high-pressure CO₂ for the hydrogenation study, EEMPA-carbamate species formed upon capture of CO₂ using EEMPA were hydrogenated in-situ. FIG. 27 shows the chemical structure of EEMPA and hydrogenation of CO₂ to different products via formamide intermediates. The results are shown in Table 7.

TABLE 7 Captured CH₄ Higher Higher Exp. CO₂ Yield C₂H₆ HC CH₄ C₂H₆ HC no. (mmol) T ° C. (%) Yield(%) Yield(%) Sel. %) Sel (%) Sel (%) 1^(a) 14.3 170 53.1 2.8 4.9 87.4 4.6 8 2^(a) 14.5 145 17.2 0.4 0.7 94 2.2 3.7 3^(a) 14.7 120 3.4 0.1 0 98 2 0 4^(a,e) 14.9 (gas) 170 80.5 0.7 0.2 99 0.8 0.2 5^(b) 14.5 170 <1 — — — — — 6^(c) 14.7 170 — — — — — — 7^(d) 14.8 170 <1 — — — — — 8^(f) 14.7 170 1.3 traces 0 97.1 2.9 — 9^(g) 14.8 170 9.7 0.6 0.4 90.2 5.9 3.8 10^(h) 15.4 170 16.4 1.1 3.2 79.2 5.2 15.6 11^(i) 14.7 170 71.2 3.1 7.4 87.1 3.8 9 P(H₂) = 15 bar, t = 3 h, EEMPA = 5 g (23 mmol), relative selectivity calculated from carbon in the gas phase. 200 mg of ^(a)5 wt % Ru/Al₂O₃ or ^(b)5 wt % Pd/ZnO/Al₂O₃, ^(c)5 wt % Pd/ZnO, ^(d)Cu/ZnO/Al₂O₃, ^(e)in the absence of EEMPA, ^(f)35 wt % Ni/alumina, ^(g)5 wt % Ru/C, ^(h)30 wt % MEA in water, ^(i)t = 6 h.

There was no detectable amount of methanol observed in the presence of Ru catalysts (Table 7). However, >50% conversion of captured CO₂ was observed with 87% selectivity for methane at 170° C. Small amounts of ethane and other higher hydrocarbons were also detected by GC. CO₂ methanation was also observed at low temperatures (145° C. and 120° C.), albeit at slower rate. Longer reaction time resulted in >90% conversion of CO₂. It is noteworthy that in the presence of capture solvent (exp. No 1), ethane and higher hydrocarbons are formed at higher selectivity than gas phase CO₂ methanation (exp. No 4). The Ru on alumina support performed better than the carbon support. Only small amounts of methane were formed in presence of Pd, Cu and Ni catalysts (Table 7). The mostly commonly used aqueous capture solvent, MEA was also screened (exp. No 10). In 30 wt % MEA solution, >20% conversion of CO₂ to hydrocarbon was observed. The CO₂ conversion is low in the aqueous solvent because the excess water in the reaction does not favor the dehydration step. This shows the importance of using water-lean solvent for this approach.

Operando NMR analysis of hydrogenation of EEMPA captured CO₂ solution in the presence of Ru/Al₂O₃showed in-situ formation of EEMPA N-formamide and simultaneous formation of methane and ethane (FIG. 28). Based on this observation, the reaction proceeds as follows: The EEMPA N-formamide (2) is formed via formate intermediate (1), which then can form many products such as methanol, methane and N-methylation depending on the catalyst and reaction conditions (FIG. 27). Gas phase CO₂ methanation typically occurs via CO intermediate. In the solution phase chemistry, there was no detectable amount of CO in both GC and NMR analysis. However, hydrogenation of metal bound CO (in-situ formed from CO₂) to methane can also take place without forming gaseous CO. To test this, N-formylpiperidine was hydrogenated to get >80% conversion of formylpiperidine to methane and piperidine without addition of external CO₂, which proved that both CO and N-formamide routes are possible in the solution phase CO₂ methanation.

In order to compare the performance of batch experiments and continuous reaction system (fixed bed) a WHSV equivalent was calculated using reaction time, solvent and catalyst loading in the batch system. Change in WHSV was achieved by changing the liquid feed flow (0.05, 0.025, 0.005 mL/min). After loading, catalyst was pretreated overnight at 120° C. under flowing H₂:N₂ mixture (50 sccm 10% v Hz). Reaction conditions: 1 g catalyst load, 870 psig (6 MPa), 38 sccm Hz, 5 sccm N₂. Two main products (methane and ethane) and two minor products (methanol and ethanol) were formed. Methane and ethane were calculated using gas chromatography; methanol and ethanol were calculated using liquid chromatography.

FIG. 29 shows that at 170° C., CO₂ conversion increased at lower WHSV (higher contact time) while selectivity towards methane remained unaffected. Methane was the main product (>86% selectivity), while ethane, methanol, and ethanol were secondary products. Methanol (and ethanol) selectivity drastically decreased at higher contact time from 7% (CO₂ WHSV=0.3 h⁻¹) to <1% (CO₂ WHSV=0.03 h⁻¹) indicating that these alcohols are reaction intermediates and that upon formation they are quickly reduced to methane (or ethane). In order to achieve higher conversion, with a 20° C. increment in reaction temperature (to 190° C., Table 8), CO₂ conversion increased to 52%, while methane selectivity remained the same.

TABLE 8 Reaction WHSV WHSV CO₂ Temp (G_(EEMPA)/ (g_(CO2)/ conversion Selectivity (% C) (° C.) g_(cat)/h⁻¹) g_(cat)/h⁻¹) (%) CH₄ C₂H₄ CH₃OH C₂H₅OH 170 3 0.3 17.6 86.7 4.8 7.2 1.2 170 1.5 0.15 35.9 89.7 7.0 2.7 0.5 170 0.3 0.03 43.2 90.1 8.5 0.4 0.9 190 3 0.3 52.1 87.7 9.1 2.7 0.6 Reaction conditions: 1 g catalyst load, 6 MPa (870 psig), 38 sccm H₂, 5 sccm N₂. Change in WHSV was achieved by changing the liquid feed flow (0.05, 0.025, 0.005 mL/min).

In view of the many possible embodiments to which the principles of the disclosed invention may be applied, it should be recognized that the illustrated embodiments are only preferred examples of the invention and should not be taken as limiting the scope of the invention. Rather, the scope of the invention is defined by the following claims. We therefore claim as our invention all that comes within the scope and spirit of these claims. 

We claim:
 1. A process, comprising: producing methane by combining a hydrogenation catalyst, H₂, and CO₂ with a condensed phase comprising an amine under conditions effective to provide a reaction between the hydrogen and CO₂ to form methane and water in the condensed phase comprising the amine, wherein at least 10 mol % of the CO₂ is consumed in the reaction; and separating the methane from the amine and the water.
 2. The process of claim 1, wherein the hydrogenation catalyst comprises Ru, Ni, Fe, Co, Rh, or a combination thereof.
 3. The process of claim 2, wherein the hydrogenation catalyst further comprises an oxide or carbon support on which the metal is disposed.
 4. The process of claim 1, wherein the conditions effective to form methane comprise: (i) a temperature T_(M) of 100° C. to 200° C.; or (ii) an initial pressure P_(M) of 0.1 MPa to 10 MPa; or (iii) a time t_(M) of 3 seconds to 36 hours; or (iv) any combination of (i), (ii), and (iii).
 5. The process of claim 1, wherein: (i) a molar ratio of H₂ to CO₂ is from 4:1 to 10:1; or (ii) the CO₂ is adsorbed, absorbed, covalently bound, or ionically bound to the amine, and the process is performed in an absence of CO₂ overpressure; or (iii) both (i) and (ii).
 6. The process of claim 1, wherein: (i) the hydrogenation catalyst is disposed in a fixed bed or fluidized bed; or (ii) the process is a continuous process; or (iii) both (i) and (ii).
 7. The process of claim 1, wherein the condensed phase is a condensed phase solution further comprising the CO₂.
 8. The process of claim 7, further comprising preparing the condensed phase solution by contacting a gas stream comprising CO₂ with an amine-containing solvent or solution prior to combining the epoxide, the hydrogenation catalyst, and the hydrogen with the condensed phase solution.
 9. The process of claim 8, wherein contacting the gas stream with the amine-containing solvent or solution is performed: (i) at a temperature T′ of 20° C. to 60° C.; or (ii) at an initial pressure P′ of 0.1 MPa to 5 MPa; or (iii) both (i) and (ii).
 10. The process of claim 1, wherein: (i) the amine comprises a 1° amine group, a 2° amine group, a 3° amine group, or any combination thereof; or (ii) wherein the CO₂ is adsorbed, absorbed, covalently bound, or ionically bound to the amine; or (iii) both (i) and (ii).
 11. The process of claim 10, wherein the amine comprises a polyamine, a tertiary amine, an aminopyridine, a compound according to Formula I, or any combination thereof, the compound according to Formula I having a structure R¹(R²)N-L¹-NH—R³ where each of R¹ and R² independently is aliphatic or cycloaliphatic or R¹ and R² together with the nitrogen to which they are attached form a heterocyclyl ring, L¹ is aliphatic or cycloaliphatic or L¹ and R¹ together with the nitrogen to which they are attached form a heterocyclic ring, and R³ is aliphatic, cycloaliphatic, cycloalkylalkyl or alkoxyalkyl.
 12. The process of claim 11, wherein the amine comprises a compound according to Formula I.
 13. The process of claim 10, wherein the amine comprises N-(2-ethoxyethyl)-3-morpholinopropan-1-amine.
 14. The process of claim 1, wherein separating the methane from the amine and the water comprises: cooling the condensed phase solution comprising the methane, the amine, and the water to a temperature of 20° C. to 50° C.; and flowing the condensed solution through a vapor-liquid separator, thereby separating the methane from the amine and the water.
 15. The process of claim 1, further comprising: separating the amine and the water; and recycling at least a portion of the amine.
 16. The process of claim 1, wherein the H₂ is present in molar excess to the CO₂, the process further comprising: separating unreacted H₂ from the methane and the amine; and recycling at least a portion of the unreacted H₂.
 17. The process of claim 1, further comprising: flowing the methane, water, and amine through a steam generator prior to separating the methane from the amine and the water, thereby generating power.
 18. The process of claim 1, wherein the hydrogenation catalyst is disposed in a fixed bed or a fluidized bed.
 19. The process of claim 1, wherein producing methane by combining the hydrogenation catalyst, H₂, and CO₂ with the condensed phase comprising the amine is a continuous process comprising flowing the H₂, the CO₂, and the condensed phase comprising the amine through a fixed or fluidized bed of the hydrogenation catalyst under the conditions effective to provide the reaction between the hydrogen and the CO₂ to form methane and water in the condensed phase comprising the amine.
 20. The process of claim 1, wherein: the condensed-phase amine comprises N-(2-ethoxyethyl)-3-morpholinopropan-1-amine (EEMPA); a concentration of CO₂ in the condensed-phase amine is 25 mol % to 50 mol %; the hydrogenation catalyst comprises 5 wt % Ru/Al₂O₃; a molar ratio of H₂ to CO₂ is from 4:1 to 10:1; and the conditions effective to form methane comprise (i) a temperature of 120° C. to 190° C., (ii) a pressure of 1 MPa to 6 MPa, and (iii) a time of 3 hours to 12 hours or a weight hourly space velocity is 0.05 g_(CO2)/g_(catalyst) h⁻¹ to 0.5 g_(CO2)/g_(catalyst) h⁻¹. 